Fuege HTML-Detailseite und HTML-Fehlerseite fuer den Status-UI-Pfad hinzu

Drei eng zusammenhaengende Aenderungen in einem Commit, da sie dieselbe
Template-Infrastruktur teilen und gemeinsam die HTML-Diagnoseansicht
abrunden:

1. CSS-Extraktion (Verbesserung, kein Verhalten geaendert)
   Das bisherige Inline-CSS wurde in die Konstante statusPageCSS
   ausgelagert. statusPageCSSBlock injiziert es per String-Konkatenation
   in alle Templates, sodass kein Template-Inheritance-Mechanismus
   benoetigt wird und jede Seite eigenstaendig ausfuehrbar bleibt.

2. GET /status/{screenId} -- neue HTML-Detailseite
   Zeigt den letzten bekannten Datensatz eines einzelnen Screens:
   Derived State, Player-Status, Connectivity und Frische als
   Summary-Cards; Timing- und Endpoints-Details in aufgeraeuemten
   Key-Value-Tabellen. Verlinkung zurueck auf /status und auf den
   bestehenden JSON-Endpunkt. Bei unbekanntem Screen: 404 mit HTML-
   Fehlerseite und Rueck-Link.
   Route: GET /status/{screenId}

3. HTML-Fehlerseite fuer /status bei ungueltigen Query-Parametern
   Bisher lieferte handleStatusPage einen rohen JSON-Fehler, wenn
   z.B. ?stale=banana uebergeben wurde -- inkongruent fuer einen
   HTML-Endpunkt. writeStatusPageQueryError rendert jetzt dasselbe
   statusPageErrorTemplate wie der Not-Found-Fall der Detailseite
   und gibt text/html mit 400 zurueck.

Neue Tests (router_test.go):
- ScreenDetailPageRoute: prueft Inhalt, Links, Content-Type
- ScreenDetailPageNotFound: prueft 404 + HTML + Rueck-Link
- StatusPageRejectsInvalidQueryParams: prueft jetzt auch text/html
  fuer alle Fehlerfaelle

Docs (PLAYER-STATUS-HTTP.md):
- Query-Parameter-Validierung mit erlaubten Werten und Fehlercodes
- Neue /status/{screenId}-Seite und HTML-Fehlerseiten dokumentiert

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/PLAYER-STATUS-HTTP.md

@@ -92,14 +92,20 @@ Zusaetzlich enthaelt die Antwort eine `summary` mit kompakten Counts fuer `total
 
 Aktuell unterstuetzte Query-Parameter fuer die Uebersicht:
 
-- `server_connectivity=<value>` zum Filtern nach Reachability-Zustand
-- `stale=true|false` zum Filtern nach serverseitiger Veraltet-Einschaetzung
-- `updated_since=<RFC3339>` zum Filtern nach `received_at`
-- `limit=<positive integer>` zum Begrenzen der Anzahl zurueckgelieferter Screens
+- `server_connectivity=<value>` zum Filtern nach Reachability-Zustand; erlaubte Werte: `online`, `offline`, `degraded`, `unknown`; ungueltige Werte liefern 400 (`invalid_server_connectivity`)
+- `stale=true|false` zum Filtern nach serverseitiger Veraltet-Einschaetzung; ungueltige Werte liefern 400 (`invalid_stale`)
+- `updated_since=<RFC3339>` zum Filtern nach `received_at`; ungueltige Zeitstempel liefern 400 (`invalid_updated_since`)
+- `limit=<positive integer>` zum Begrenzen der Anzahl zurueckgelieferter Screens; nicht-positive Werte liefern 400 (`invalid_limit`)
 
 Die Query-Parameter beeinflussen die Liste in `screens`; die `summary` beschreibt weiterhin den gesamten aktuell bekannten Statusbestand.
 Dieselben Parameter koennen aktuell sowohl an `GET /api/v1/screens/status` als auch an `GET /status` verwendet werden, damit Browser-Ansicht und JSON-Uebersicht dieselbe Diagnose-Sicht teilen.
 
+`GET /status/{screenId}` liefert eine HTML-Detailseite fuer einen einzelnen Screen.
+Sie zeigt denselben Datensatz wie der JSON-Endpunkt – Derived State, Player-Status, Connectivity, Frische, Timestamps und Endpoints – in derselben visuellen Sprache wie die Uebersichtsseite.
+Bei unbekanntem Screen liefert sie 404 mit einer erklaerenden HTML-Fehlermeldung und einem Rueck-Link auf `/status`.
+
+Fehlerfall bei ungueltigem Query-Parameter auf `/status` (z.B. `?stale=banana`): statt rohem JSON liefert der Endpunkt jetzt eine HTML-Fehlerseite mit erklaerenden Hinweisen und einem Rueck-Link.
+
 `GET /api/v1/screens/{screenId}/status` liefert den zuletzt akzeptierten Status fuer einen einzelnen Screen zurueck.
 Wenn fuer den Screen noch kein Status vorliegt, liefert das Backend `404` mit dem gemeinsamen Fehlerumschlag.
 

server/backend/internal/httpapi/router.go

@@ -15,6 +15,7 @@ func NewRouter(store playerStatusStore) http.Handler {
 	})
 
 	mux.HandleFunc("GET /status", handleStatusPage(store))
+	mux.HandleFunc("GET /status/{screenId}", handleScreenDetailPage(store))
 
 	mux.HandleFunc("GET /api/v1", func(w http.ResponseWriter, _ *http.Request) {
 		writeJSON(w, http.StatusOK, map[string]any{

server/backend/internal/httpapi/router_test.go

@@ -176,6 +176,74 @@ func TestRouterScreenStatusListRoute(t *testing.T) {
 	}
 }
 
+func TestRouterScreenDetailPageRoute(t *testing.T) {
+	store := newInMemoryPlayerStatusStore()
+	store.now = func() time.Time {
+		return time.Date(2026, 3, 22, 16, 10, 0, 0, time.UTC)
+	}
+	store.Save(playerStatusRecord{
+		ScreenID:              "info01-dev",
+		Timestamp:             "2026-03-22T16:09:30Z",
+		Status:                "running",
+		ServerConnectivity:    "online",
+		ReceivedAt:            "2026-03-22T16:09:30Z",
+		HeartbeatEverySeconds: 30,
+		ServerURL:             "http://127.0.0.1:8080",
+		MQTTBroker:            "tcp://127.0.0.1:1883",
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/status/info01-dev", nil)
+	w := httptest.NewRecorder()
+
+	NewRouter(store).ServeHTTP(w, req)
+
+	if got, want := w.Code, http.StatusOK; got != want {
+		t.Fatalf("status = %d, want %d", got, want)
+	}
+
+	if got := w.Header().Get("Content-Type"); !strings.Contains(got, "text/html") {
+		t.Fatalf("Content-Type = %q, want text/html", got)
+	}
+
+	body := w.Body.String()
+	for _, want := range []string{
+		"info01-dev",
+		"online",
+		"running",
+		"fresh",
+		"http://127.0.0.1:8080",
+		"tcp://127.0.0.1:1883",
+		"2026-03-22T16:09:30Z",
+		"/api/v1/screens/info01-dev/status",
+		"← All screens",
+		"Timing",
+		"Endpoints",
+	} {
+		if !strings.Contains(body, want) {
+			t.Fatalf("body missing %q", want)
+		}
+	}
+}
+
+func TestRouterScreenDetailPageNotFound(t *testing.T) {
+	req := httptest.NewRequest(http.MethodGet, "/status/missing-screen", nil)
+	w := httptest.NewRecorder()
+
+	NewRouter(newInMemoryPlayerStatusStore()).ServeHTTP(w, req)
+
+	if got, want := w.Code, http.StatusNotFound; got != want {
+		t.Fatalf("status = %d, want %d", got, want)
+	}
+
+	if got := w.Header().Get("Content-Type"); !strings.Contains(got, "text/html") {
+		t.Fatalf("Content-Type = %q, want text/html", got)
+	}
+
+	if !strings.Contains(w.Body.String(), "← Back to Screen Status") {
+		t.Fatal("body missing back link")
+	}
+}
+
 func TestRouterStatusPageRejectsInvalidQueryParams(t *testing.T) {
 	cases := []struct {
 		name  string
@@ -198,6 +266,9 @@ func TestRouterStatusPageRejectsInvalidQueryParams(t *testing.T) {
 			if got, want := w.Code, http.StatusBadRequest; got != want {
 				t.Fatalf("status = %d, want %d", got, want)
 			}
+			if got := w.Header().Get("Content-Type"); !strings.Contains(got, "text/html") {
+				t.Fatalf("Content-Type = %q, want text/html (error page must be HTML, not JSON)", got)
+			}
 		})
 	}
 }

server/backend/internal/httpapi/statuspage.go

@@ -32,19 +32,21 @@ type statusFilterLink struct {
 	Active bool
 }
 
-var statusPageTemplate = template.Must(template.New("status-page").Funcs(template.FuncMap{
-	"connectivityLabel": connectivityLabel,
-	"screenDetailPath":  screenDetailPath,
-	"statusClass":       statusClass,
-	"timestampLabel":    timestampLabel,
-}).Parse(`<!DOCTYPE html>
-<html lang="en">
-<head>
-  <meta charset="utf-8">
-  <meta name="viewport" content="width=device-width, initial-scale=1">
-  <meta http-equiv="refresh" content="{{.RefreshSeconds}}">
-  <title>Screen Status</title>
-  <style>
+type screenDetailPageData struct {
+	GeneratedAt    string
+	Record         playerStatusRecord
+	StatusPagePath string
+}
+
+type statusPageErrorData struct {
+	Message        string
+	StatusPagePath string
+}
+
+// statusPageCSS is the shared stylesheet for all status page templates.
+// It is injected via string concatenation at parse time so that each template
+// is self-contained and can be executed independently without template inheritance.
+const statusPageCSS = `
     :root {
       color-scheme: light;
       --bg: #f4efe7;
@@ -366,6 +368,23 @@ var statusPageTemplate = template.Must(template.New("status-page").Funcs(templat
       line-height: 1.5;
     }
 
+    /* Key-value detail tables used on the screen detail page. */
+    .detail-table {
+      min-width: 0;
+    }
+
+    .detail-table th {
+      font-weight: normal;
+      white-space: nowrap;
+      padding-right: 24px;
+      width: 40%;
+    }
+
+    .detail-table tbody tr:last-child th,
+    .detail-table tbody tr:last-child td {
+      border-bottom: 0;
+    }
+
     @media (max-width: 720px) {
       .page {
         padding: 20px 14px 32px;
@@ -392,8 +411,34 @@ var statusPageTemplate = template.Must(template.New("status-page").Funcs(templat
       .filter-form {
         grid-template-columns: 1fr;
       }
+
+      .detail-table th {
+        width: auto;
       }
-  </style>
+    }
+`
+
+// statusPageCSSBlock wraps statusPageCSS in a <style> element.
+// Each template embeds it via string concatenation so that no template
+// inheritance is needed and every page is independently executable.
+const statusPageCSSBlock = `  <style>` + statusPageCSS + `  </style>`
+
+var statusTemplateFuncs = template.FuncMap{
+	"connectivityLabel":    connectivityLabel,
+	"screenDetailPath":     screenDetailPath,
+	"screenDetailHTMLPath": screenDetailHTMLPath,
+	"statusClass":          statusClass,
+	"timestampLabel":       timestampLabel,
+}
+
+var statusPageTemplate = template.Must(template.New("status-page").Funcs(statusTemplateFuncs).Parse(`<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta http-equiv="refresh" content="{{.RefreshSeconds}}">
+  <title>Screen Status</title>
+` + statusPageCSSBlock + `
 </head>
 <body>
   <main class="page">
@@ -495,7 +540,7 @@ var statusPageTemplate = template.Must(template.New("status-page").Funcs(templat
       <div class="panel-head">
         <div>
           <h2>Latest reports</h2>
-          <p class="panel-copy">Each row links directly to the existing per-screen JSON detail endpoint for a quick drill-down into the raw status payload.</p>
+          <p class="panel-copy">Each row links to the HTML detail view and the raw JSON endpoint for a quick drill-down.</p>
         </div>
       </div>
       <div class="table-actions">
@@ -521,7 +566,8 @@ var statusPageTemplate = template.Must(template.New("status-page").Funcs(templat
                 <div class="screen">{{.ScreenID}}</div>
                 {{if .MQTTBroker}}<small>{{.MQTTBroker}}</small>{{else if .ServerURL}}<small>{{.ServerURL}}</small>{{else}}<small>No endpoint details</small>{{end}}
                 <div class="screen-links">
-                  <a class="json-link" href="{{screenDetailPath .ScreenID}}">JSON detail</a>
+                  <a class="filter-link" href="{{screenDetailHTMLPath .ScreenID}}">Details</a>
+                  <a class="json-link" href="{{screenDetailPath .ScreenID}}">JSON</a>
                 </div>
               </td>
               <td><span class="pill {{statusClass .DerivedState}}">{{.DerivedState}}</span></td>
@@ -555,11 +601,138 @@ var statusPageTemplate = template.Must(template.New("status-page").Funcs(templat
 </html>
 `))
 
+var screenDetailTemplate = template.Must(template.New("screen-detail").Funcs(statusTemplateFuncs).Parse(`<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>{{.Record.ScreenID}} – Screen Status</title>
+` + statusPageCSSBlock + `
+</head>
+<body>
+  <main class="page">
+    <section class="hero">
+      <div class="hero-top">
+        <div>
+          <h1>{{.Record.ScreenID}}</h1>
+          <p class="lead">Single screen diagnostic view based on the last accepted status report.</p>
+        </div>
+        <div class="meta">
+          <div>Updated {{.GeneratedAt}}</div>
+          <div style="margin-top: 8px; display: flex; gap: 8px; flex-wrap: wrap; justify-content: flex-end;">
+            <a class="meta-chip" href="{{.StatusPagePath}}">← All screens</a>
+            <a class="meta-chip" href="{{screenDetailPath .Record.ScreenID}}">JSON</a>
+          </div>
+        </div>
+      </div>
+
+      <div class="summary-grid">
+        <article class="summary-card {{statusClass .Record.DerivedState}}">
+          <strong><span class="state {{statusClass .Record.DerivedState}}">{{.Record.DerivedState}}</span></strong>
+          <span>Derived state</span>
+        </article>
+        <article class="summary-card">
+          <strong>{{.Record.Status}}</strong>
+          <span>Player status</span>
+        </article>
+        <article class="summary-card {{statusClass .Record.ServerConnectivity}}">
+          <strong>{{connectivityLabel .Record.ServerConnectivity}}</strong>
+          <span>Server connectivity</span>
+        </article>
+        <article class="summary-card">
+          <strong>{{if .Record.Stale}}stale{{else}}fresh{{end}}</strong>
+          <span>Freshness</span>
+        </article>
+      </div>
+    </section>
+
+    <section class="panel">
+      <div class="panel-head">
+        <div>
+          <h2>Timing</h2>
+          <p class="panel-copy">Timestamps reported by the player and annotated by the server at receive time.</p>
+        </div>
+      </div>
+      <table class="detail-table">
+        <tbody>
+          <tr>
+            <th>Received at (server)</th>
+            <td>{{timestampLabel .Record.ReceivedAt}}</td>
+          </tr>
+          <tr>
+            <th>Player timestamp</th>
+            <td>{{timestampLabel .Record.Timestamp}}</td>
+          </tr>
+          <tr>
+            <th>Started at</th>
+            <td>{{timestampLabel .Record.StartedAt}}</td>
+          </tr>
+          <tr>
+            <th>Last heartbeat at</th>
+            <td>{{timestampLabel .Record.LastHeartbeatAt}}</td>
+          </tr>
+          <tr>
+            <th>Heartbeat interval</th>
+            <td>{{if gt .Record.HeartbeatEverySeconds 0}}{{.Record.HeartbeatEverySeconds}}s{{else}}-{{end}}</td>
+          </tr>
+        </tbody>
+      </table>
+    </section>
+
+    <section class="panel">
+      <div class="panel-head">
+        <div>
+          <h2>Endpoints</h2>
+          <p class="panel-copy">Connection details reported by the player in the last accepted status.</p>
+        </div>
+      </div>
+      <table class="detail-table">
+        <tbody>
+          <tr>
+            <th>Server URL</th>
+            <td>{{if .Record.ServerURL}}{{.Record.ServerURL}}{{else}}-{{end}}</td>
+          </tr>
+          <tr>
+            <th>MQTT broker</th>
+            <td>{{if .Record.MQTTBroker}}{{.Record.MQTTBroker}}{{else}}-{{end}}</td>
+          </tr>
+        </tbody>
+      </table>
+    </section>
+  </main>
+</body>
+</html>
+`))
+
+var statusPageErrorTemplate = template.Must(template.New("status-error").Funcs(statusTemplateFuncs).Parse(`<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Invalid filter – Screen Status</title>
+` + statusPageCSSBlock + `
+</head>
+<body>
+  <main class="page" style="max-width: 640px;">
+    <section class="hero">
+      <div class="hero-top">
+        <div>
+          <h1>Invalid filter</h1>
+          <p class="lead">{{.Message}}</p>
+        </div>
+      </div>
+      <a class="filter-link" href="{{.StatusPagePath}}">← Back to Screen Status</a>
+    </section>
+  </main>
+</body>
+</html>
+`))
+
 func handleStatusPage(store playerStatusStore) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		overview, err := buildScreenStatusOverview(store, r.URL.Query())
 		if err != nil {
-			writeOverviewQueryError(w, err)
+			writeStatusPageQueryError(w, err)
 			return
 		}
 
@@ -574,6 +747,40 @@ func handleStatusPage(store playerStatusStore) http.HandlerFunc {
 	}
 }
 
+func handleScreenDetailPage(store playerStatusStore) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		screenID := strings.TrimSpace(r.PathValue("screenId"))
+
+		record, ok := store.Get(screenID)
+		if !ok {
+			data := statusPageErrorData{
+				Message:        "Fuer diesen Screen liegt noch kein Status vor.",
+				StatusPagePath: "/status",
+			}
+			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			w.WriteHeader(http.StatusNotFound)
+			_ = statusPageErrorTemplate.Execute(w, data)
+			return
+		}
+
+		record.Stale = isStale(record, store.Now())
+		record.DerivedState = deriveState(record)
+
+		data := screenDetailPageData{
+			GeneratedAt:    store.Now().Format(time.RFC3339),
+			Record:         record,
+			StatusPagePath: "/status",
+		}
+
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+
+		if err := screenDetailTemplate.Execute(w, data); err != nil {
+			http.Error(w, "failed to render detail page", http.StatusInternalServerError)
+		}
+	}
+}
+
 func buildStatusPageData(store playerStatusStore, query url.Values, overview screenStatusOverview) statusPageData {
 	filters := statusPageFilters{
 		ServerConnectivity: strings.TrimSpace(query.Get("server_connectivity")),
@@ -653,6 +860,39 @@ func buildOverviewPath(basePath string, filters statusPageFilters) string {
 	return basePath + "?" + encoded
 }
 
+// writeStatusPageQueryError renders an HTML 400 error page for invalid query
+// parameters on /status. This keeps the error in the same visual context as
+// the rest of the status UI instead of returning a raw JSON blob in a browser.
+func writeStatusPageQueryError(w http.ResponseWriter, queryErr error) {
+	data := statusPageErrorData{
+		Message:        overviewQueryErrorMessage(queryErr),
+		StatusPagePath: "/status",
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.WriteHeader(http.StatusBadRequest)
+	if err := statusPageErrorTemplate.Execute(w, data); err != nil {
+		http.Error(w, data.Message, http.StatusBadRequest)
+	}
+}
+
+// overviewQueryErrorMessage returns a human-readable message for the given
+// overview query validation error. It is shared between the HTML and JSON
+// error paths to keep messages consistent.
+func overviewQueryErrorMessage(err error) string {
+	switch err {
+	case errInvalidUpdatedSince:
+		return "updated_since ist kein gueltiger RFC3339-Zeitstempel."
+	case errInvalidLimit:
+		return "limit muss eine positive Ganzzahl sein."
+	case errInvalidServerConnectivity:
+		return "server_connectivity muss online, offline, degraded oder unknown sein."
+	case errInvalidStale:
+		return "stale muss true oder false sein."
+	default:
+		return "Ungueltige Query-Parameter."
+	}
+}
+
 func statusClass(value string) string {
 	trimmed := strings.TrimSpace(value)
 	if trimmed == "" {
@@ -673,6 +913,10 @@ func screenDetailPath(screenID string) string {
 	return "/api/v1/screens/" + url.PathEscape(strings.TrimSpace(screenID)) + "/status"
 }
 
+func screenDetailHTMLPath(screenID string) string {
+	return "/status/" + url.PathEscape(strings.TrimSpace(screenID))
+}
+
 func timestampLabel(value string) string {
 	trimmed := strings.TrimSpace(value)
 	if trimmed == "" {