feat(web): stream resilience — preload=auto, stall recovery, next-track prefetch

Phases 1+2 of the background-play resilience work. Net effect on
desktop: gap-free track transitions on slow networks, automatic
recovery from transient stalls / network errors mid-track.

- preload="metadata" → "auto" on the main audio element so the
  browser pre-buffers more aggressively.
- attachStallRetry() — listens for stalled / waiting / network-error
  events. Schedules a reload-from-current-position after delayMs
  (3s default) if the buffer hasn't recovered. MEDIA_ERR_NETWORK
  triggers immediate retry. Bounded by maxRetries (3 default) to
  prevent tight loops on persistent failures.
- audioLoader seam — `lib/player/audioLoader.ts` exposes a
  resolve(track) → URL + optional prefetch(track) hint. Default
  returns track.stream_url; the planned Tauri shell can swap via
  setAudioLoader() to return blob URLs backed by a Rust cache,
  mirroring the Flutter drift+LockCachingAudioSource pattern.
- Hidden prefetch <audio> element. When current track passes 50%,
  start loading queue[index+1]. On track-end the swap is gap-free
  via browser HTTP cache + audio buffer warm-up.

Phase 3 (service worker chunk cache) deliberately deferred — that
work belongs in the Tauri shell as native Rust caching, not as a
web-only investment that would compete with it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/player/audioLoader.test.ts

@@ -0,0 +1,52 @@
+import { afterEach, describe, expect, test, vi } from 'vitest';
+import type { TrackRef } from '$lib/api/types';
+import { audioLoader, setAudioLoader, __resetAudioLoader } from './audioLoader';
+
+const sampleTrack: TrackRef = {
+  id: 't1',
+  title: 'Sample',
+  album_id: 'a1',
+  album_title: 'Album',
+  artist_id: 'ar1',
+  artist_name: 'Artist',
+  duration_sec: 180,
+  stream_url: '/api/tracks/t1/stream'
+};
+
+afterEach(() => {
+  __resetAudioLoader();
+});
+
+describe('audioLoader', () => {
+  test('default resolve returns the track stream_url', () => {
+    expect(audioLoader.resolve(sampleTrack)).toBe('/api/tracks/t1/stream');
+  });
+
+  test('default prefetch is a no-op (does not throw)', () => {
+    expect(() => audioLoader.prefetch?.(sampleTrack)).not.toThrow();
+  });
+
+  test('setAudioLoader swaps the resolver', () => {
+    setAudioLoader({
+      resolve: (t) => `blob:cached-${t.id}`
+    });
+    expect(audioLoader.resolve(sampleTrack)).toBe('blob:cached-t1');
+  });
+
+  test('setAudioLoader swaps the prefetch hook', () => {
+    const spy = vi.fn();
+    setAudioLoader({
+      resolve: (t) => t.stream_url,
+      prefetch: spy
+    });
+    audioLoader.prefetch?.(sampleTrack);
+    expect(spy).toHaveBeenCalledWith(sampleTrack);
+  });
+
+  test('__resetAudioLoader restores the default', () => {
+    setAudioLoader({ resolve: () => 'blob:swapped' });
+    expect(audioLoader.resolve(sampleTrack)).toBe('blob:swapped');
+    __resetAudioLoader();
+    expect(audioLoader.resolve(sampleTrack)).toBe('/api/tracks/t1/stream');
+  });
+});