feat(player): webview audio backend for platforms without a native one

Adds WebviewAudioBackend, used on non-Linux/non-Android targets (e.g.
Windows) where there is no libmpv/ExoPlayer. Instead of decoding, it
emits a WebviewAudioLoad event with the stream URL; a frontend <audio>
element (WebviewAudioAdapter + webviewAudio service) plays it and reports
state/position back through the existing player_report_* round-trip, so
the Rust PlayerController stays the single source of truth. Play/pause/
seek reach the element via the existing ControlCommand event.

All video already renders in the webview on every platform, so this
completes audio-only playback for Windows (video via WebView2, audio via
<audio>). Pure Rust + Tauri events, so it still cross-compiles from Linux.

Regenerates bindings.ts (adds webview_audio_load; also carries the
equalizer EQ bindings).

TRACES: UR-003, UR-004, UR-005 | DR-004

src/routes/+layout.svelte

@@ -7,6 +7,7 @@
   import { auth, needsReauth, isAuthenticated } from "$lib/stores/auth";
   import { connectivity, isConnected } from "$lib/stores/connectivity";
   import { initPlayerEvents, cleanupPlayerEvents } from "$lib/services/playerEvents";
+  import { initWebviewAudio, cleanupWebviewAudio } from "$lib/services/webviewAudio";
   import { downloads, initDownloadEvents, cleanupDownloadEvents } from "$lib/stores/downloads";
   import { syncService } from "$lib/services/syncService";
   import { onReconnected as onCatalogReconnected, syncCatalog, refreshSyncStatus, showServerCatalog, lastCatalogSync } from "$lib/services/offlineCatalog";
@@ -86,6 +87,11 @@
     // Initialize player event listener for push-based updates
     await initPlayerEvents();
 
+    // Initialize the webview audio controller (plays audio-only media in an
+    // <audio> element on platforms with no native audio backend, e.g. Windows;
+    // self-gates and is a no-op on Linux/Android).
+    await initWebviewAudio();
+
     // Initialize download event listener
     await initDownloadEvents();
 
@@ -122,6 +128,7 @@
   onDestroy(() => {
     stopNetworkReporting?.();
     cleanupPlayerEvents();
+    cleanupWebviewAudio();
     cleanupDownloadEvents();
     connectivity.stopMonitoring();
     syncService.stop();