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/lib/services/webviewAudio.ts

@@ -0,0 +1,109 @@
+/**
+ * Webview audio controller — the frontend half of audio-only playback on
+ * platforms with no native audio backend (currently Windows).
+ *
+ * The Rust `WebviewAudioBackend` emits a `webview_audio_load` event carrying the
+ * stream URL whenever a track loads. This controller owns a single hidden
+ * `<audio>` element, plays that URL through a {@link WebviewAudioAdapter}, and
+ * registers the adapter with the player facade so backend `control_command`
+ * events (play/pause/seek — routed by playerEvents.ts) reach the element. The
+ * adapter reports state/position back through the standard `player_report_*`
+ * round-trip, keeping the Rust controller the single source of truth.
+ *
+ * No-op on platforms with a native audio backend (Linux/Android): the backend
+ * never emits `webview_audio_load` there, so even if initialized this listener
+ * stays idle. We still gate initialization on platform to avoid mounting a stray
+ * element.
+ *
+ * TRACES: UR-003, UR-005 | DR-004
+ */
+
+import { type UnlistenFn } from "@tauri-apps/api/event";
+import { events } from "$lib/api/bindings";
+import { playerController } from "$lib/player";
+import { createRustReportHost } from "$lib/player/adapters/rustReportHost";
+import { WebviewAudioAdapter } from "$lib/player/adapters/webviewAudioAdapter";
+
+let unlisten: UnlistenFn | null = null;
+let audioEl: HTMLAudioElement | null = null;
+let adapter: WebviewAudioAdapter | null = null;
+
+/** Platforms whose Rust backend renders audio in the webview rather than natively. */
+function usesWebviewAudio(): boolean {
+  // Native audio backends exist only for Linux (mpv) and Android (ExoPlayer).
+  // Everything else (Windows, and any future desktop) uses the webview element.
+  // We detect "not linux/android" rather than "is windows" so new desktop
+  // targets are covered automatically, matching the Rust cfg gate.
+  if (typeof navigator === "undefined") return false;
+  const ua = navigator.userAgent.toLowerCase();
+  const isAndroid = ua.includes("android");
+  const isLinux = ua.includes("linux") && !isAndroid;
+  return !isAndroid && !isLinux;
+}
+
+/**
+ * Initialize the webview audio controller. Safe to call unconditionally from the
+ * root layout; it self-gates on platform and is idempotent.
+ */
+export async function initWebviewAudio(): Promise<void> {
+  if (unlisten) return;
+  if (!usesWebviewAudio()) return;
+
+  audioEl = document.createElement("audio");
+  audioEl.hidden = true;
+  audioEl.preload = "auto";
+  // Kept in the DOM so the browser keeps decoding it when not focused.
+  document.body.appendChild(audioEl);
+
+  unlisten = await events.playerStatusEvent.listen((event) => {
+    const p = event.payload;
+    if (p.type !== "webview_audio_load") return;
+    void handleLoad(p.url, p.media_id, p.position, p.autoplay);
+  });
+}
+
+async function handleLoad(
+  url: string,
+  mediaId: string | null,
+  position: number,
+  autoplay: boolean
+): Promise<void> {
+  if (!audioEl) return;
+
+  // Fresh host/adapter per load so reporting targets the current media id.
+  const host = createRustReportHost(mediaId ?? "", {});
+  adapter = new WebviewAudioAdapter(audioEl, host);
+  playerController.setActiveAdapter(adapter);
+
+  await adapter.load(url, {
+    mediaId: mediaId ?? "",
+    mediaSourceId: null,
+    needsTranscoding: false,
+    initialPosition: position,
+    isLive: false,
+    audioTrackIndex: null,
+    knownDuration: 0,
+    subtitleTracks: [],
+  });
+
+  if (!autoplay) {
+    await adapter.pause();
+  }
+}
+
+/** Tear down the controller (idempotent). */
+export function cleanupWebviewAudio(): void {
+  if (unlisten) {
+    unlisten();
+    unlisten = null;
+  }
+  if (adapter) {
+    playerController.clearActiveAdapter(adapter);
+    void adapter.dispose();
+    adapter = null;
+  }
+  if (audioEl) {
+    audioEl.remove();
+    audioEl = null;
+  }
+}