feat(offline): make the web UI usable without a reachable backend

Three tiers of offline support, all scoped to the active backend's
localStorage namespace (mirroring the auth slice):

Tier 1 — persist client state. queue + player slices are saved (queue
entries/index/source; player track/position/volume/repeat/shuffle) and
rehydrated on load, so a reload with no backend restores where the user
left off. Playback never auto-resumes (browsers block autoplay). Retires
the DEMO_QUEUE and isQueueOpen:true stubs.

Tier 2 — persist the RTK Query cache. Last-seen library/albums/artists
are snapshotted (fulfilled queries only) and replayed via RTKQ's
extractRehydrationInfo at startup, so the library renders read-only when
the backend is down. ConnectionStatus tooltip flags cached data offline.
No server data is copied into a slice — the cache feeds itself back.

Tier 3 — service worker audio + cover cache (PWA). Audio streams are
cached keyed by content id (token stripped), range-aware (synthetic 206
slicing), with a 500MB LRU cap, so already-played tracks play fully
offline. Cover art uses stale-while-revalidate in its own bounded cache.
Module worker (ESM); pure helpers split into sw-core.js and unit-tested.
Web app manifest enables "Install app". Player source badge now reflects
real cached state.

tsc clean, lint clean, 19 new tests pass, production build verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/player/PersistentPlayer.tsx

@@ -14,6 +14,7 @@ import {
   toggleQueue,
 } from '../../store/slices/player';
 import { useAudioPlayer } from '../../hooks/useAudioPlayer';
+import { useStreamCached } from '../../hooks/useStreamCached';
 import { formatDuration } from '../../lib/format';
 import { getCoverUrl } from '../../api/endpoints/streaming';
 
@@ -24,6 +25,8 @@ export function PersistentPlayer() {
   const player = useAppSelector((s) => s.player);
   const queue = useAppSelector((s) => s.queue);
   const currentEntry = queue.entries[queue.currentIndex];
+  // Source indicator: cached → playing locally, otherwise streaming.
+  const cached = useStreamCached(currentEntry?.trackId);
 
   if (!currentEntry && !player.currentTrackId) {
     return <div className="player empty">{t('player.nothingPlaying')}</div>;
@@ -31,7 +34,7 @@ export function PersistentPlayer() {
 
   const artUrl = getCoverUrl(currentEntry?.albumArtUrl);
   const seedLabel = currentEntry?.albumTitle ?? currentEntry?.title ?? '';
-  const onStream = true;
+  const onStream = !cached;
 
   return (
     <div className="player">