refactor(web): serve the SPA under /app/* (refs #451)

Move the soundtouch-web single-page app from top-level page paths
(/devices, /tunein, ...) under one /app subtree, so the whole web UI
lives under /app/* and folding -web into -service stays an additive
mount. The client navigates via component state rather than the URL and
all assets are referenced absolutely (/static/...), so this is a pure
routing change: no frontend edits needed.

The bare root / now redirects into the app (standalone convenience).
When -web is folded into -service, / instead serves a landing page
(admin vs app) and this redirect is replaced.

Extend mount_test.go with TestMountSPARoutes: the SPA resolves under
/app, the old top-level page paths are gone, and / remains only as the
redirect.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/service/soundtouchweb/mount.go

@@ -99,14 +99,25 @@ func (app *WebApp) Mount(r chi.Router, discoveryService *discovery.UnifiedDiscov
 		})
 	})
 
-	// SPA routes — serve index.html for client-side routing
-	r.Get("/", app.serveIndex)
-	r.Get("/devices", app.serveIndex)
-	r.Get("/device/*", app.serveIndex)
-	r.Get("/tunein", app.serveIndex)
-	r.Get("/radiobrowser", app.serveIndex)
-	r.Get("/playurl", app.serveIndex)
-	r.Get("/tts", app.serveIndex)
+	// SPA — served under /app/*. The client navigates via component state
+	// rather than the URL, so these entries only ensure deep links and
+	// refreshes return index.html instead of 404. Per #451 this keeps the
+	// whole web UI under one /app subtree, so folding -web into -service is an
+	// additive mount.
+	r.Get("/app", app.serveIndex)
+	r.Get("/app/devices", app.serveIndex)
+	r.Get("/app/device/*", app.serveIndex)
+	r.Get("/app/tunein", app.serveIndex)
+	r.Get("/app/radiobrowser", app.serveIndex)
+	r.Get("/app/playurl", app.serveIndex)
+	r.Get("/app/tts", app.serveIndex)
+
+	// Standalone convenience: the bare root jumps into the app. When -web is
+	// folded into -service, / instead serves a landing page (admin vs app) and
+	// this redirect is replaced.
+	r.Get("/", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/app", http.StatusFound)
+	})
 }
 
 func (app *WebApp) serveIndex(w http.ResponseWriter, _ *http.Request) {

pkg/service/soundtouchweb/mount_test.go

@@ -59,3 +59,42 @@ func TestMountControlAPIShape(t *testing.T) {
 		t.Errorf("expected /api/control/version to be registered; got %v", apiRoutes)
 	}
 }
+
+// TestMountSPARoutes verifies the issue #451 SPA move: the web UI is served
+// under /app/* and the old top-level page paths are gone, with / kept only as
+// a redirect into the app.
+func TestMountSPARoutes(t *testing.T) {
+	app := NewWebApp()
+
+	r := chi.NewRouter()
+	app.Mount(r, nil)
+
+	routes := map[string]bool{}
+
+	walkErr := chi.Walk(r, func(_, route string, _ http.Handler, _ ...func(http.Handler) http.Handler) error {
+		routes[route] = true
+
+		return nil
+	})
+	if walkErr != nil {
+		t.Fatalf("walk routes: %v", walkErr)
+	}
+
+	for _, want := range []string{"/app", "/app/devices", "/app/tunein"} {
+		if !routes[want] {
+			t.Errorf("expected SPA route %q under /app to be registered", want)
+		}
+	}
+
+	// The old top-level page paths moved under /app.
+	for _, gone := range []string{"/devices", "/device/*", "/tunein", "/radiobrowser", "/playurl", "/tts"} {
+		if routes[gone] {
+			t.Errorf("top-level SPA route %q should have moved under /app", gone)
+		}
+	}
+
+	// / stays registered, but only as the redirect into the app.
+	if !routes["/"] {
+		t.Error("expected / to remain registered (redirect into the app)")
+	}
+}