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/store/rtkqPersist.ts

@@ -0,0 +1,76 @@
+/*
+ * Tier 2 offline support: persist the RTK Query cache (last-seen server data —
+ * library/albums/artists/…) to the active backend's localStorage namespace and
+ * replay it into the cache on startup. With the backend down, components keep
+ * rendering the last-known data read-only instead of an error state.
+ *
+ * Per the architecture invariant, server data is NOT copied into a slice: this
+ * snapshots the RTKQ cache itself and feeds it back through RTKQ's own
+ * `extractRehydrationInfo` mechanism (see `api/index.ts`).
+ */
+import { instanceStorage } from '../config/instances';
+import { rehydrateApi, type RehydrateApiPayload } from '../api/rehydrate';
+import type { RootState } from './index';
+
+const CACHE_KEY = 'rtkq';
+
+type ApiState = RootState['api'];
+type QueryEntry = ApiState['queries'][string];
+
+/**
+ * Keep only successfully-fulfilled query results — pending/rejected entries
+ * carry no usable data and subscriptions are rebuilt by components on mount.
+ * Mutation results are never restored.
+ */
+function snapshot(apiState: ApiState): RehydrateApiPayload {
+  const queries: Record<string, unknown> = {};
+  for (const [key, entry] of Object.entries(apiState.queries)) {
+    const q = entry as QueryEntry | undefined;
+    if (q && q.status === 'fulfilled') queries[key] = q;
+  }
+  return { queries, mutations: {} };
+}
+
+function load(): RehydrateApiPayload | null {
+  try {
+    const raw = instanceStorage.get(CACHE_KEY);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw) as Partial<RehydrateApiPayload>;
+    if (!parsed.queries) return null;
+    return { queries: parsed.queries, mutations: {} };
+  } catch {
+    return null;
+  }
+}
+
+/** Replay the persisted cache into RTKQ. Call once after the store is created. */
+export function rehydrateApiCache(dispatch: (action: unknown) => void): void {
+  const cached = load();
+  if (cached) dispatch(rehydrateApi(cached));
+}
+
+/**
+ * Subscribe a store so the RTKQ cache is flushed to localStorage. Throttled,
+ * since cache state churns on every in-flight query transition.
+ */
+export function startApiPersistence(store: {
+  getState: () => RootState;
+  subscribe: (listener: () => void) => () => void;
+}): () => void {
+  let last = '';
+  let timer: ReturnType<typeof setTimeout> | null = null;
+
+  const flush = () => {
+    timer = null;
+    const snap = JSON.stringify(snapshot(store.getState().api));
+    if (snap !== last) {
+      instanceStorage.set(CACHE_KEY, snap);
+      last = snap;
+    }
+  };
+
+  return store.subscribe(() => {
+    if (timer) return;
+    timer = setTimeout(flush, 2000);
+  });
+}