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/rehydrate.ts

@@ -0,0 +1,20 @@
+import { createAction } from '@reduxjs/toolkit';
+
+/*
+ * Tier 2 offline support contract. RTK Query can seed its cache from a
+ * persisted snapshot via `extractRehydrationInfo` (see `api/index.ts`). We use
+ * a single action whose payload is the previously-saved api slice state; the
+ * api reducer pulls `queries`/`mutations` out of it on startup so last-seen
+ * library data renders read-only while the backend is unreachable.
+ *
+ * The type string lives here (not in the store) so `api/index.ts` can match it
+ * without importing from the store layer (which would create a cycle).
+ */
+export const REHYDRATE_API = 'api/rehydrate';
+
+export interface RehydrateApiPayload {
+  queries: Record<string, unknown>;
+  mutations: Record<string, unknown>;
+}
+
+export const rehydrateApi = createAction<RehydrateApiPayload>(REHYDRATE_API);