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/api/index.ts

@@ -1,5 +1,6 @@
 import { createApi } from '@reduxjs/toolkit/query/react';
 import { baseQueryWithReauth } from './baseQuery';
+import { REHYDRATE_API, type RehydrateApiPayload } from './rehydrate';
 
 export const api = createApi({
   reducerPath: 'api',
@@ -14,5 +15,16 @@ export const api = createApi({
     'User',
     'Storage',
   ],
+  // Tier 2 offline: seed the cache from the persisted snapshot dispatched at
+  // startup (see `store/rtkqPersist.ts`). Returning the saved queries/mutations
+  // lets the last-seen library render before — or instead of — any network call.
+  extractRehydrationInfo(action) {
+    if (action.type === REHYDRATE_API) {
+      // The api reducer reads `queries`/`mutations` off this and restores any
+      // fulfilled entries; pending/rejected ones are ignored automatically.
+      return action.payload as RehydrateApiPayload as never;
+    }
+    return undefined;
+  },
   endpoints: () => ({}),
 });