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>

tests/rtkqPersist.test.ts

@@ -0,0 +1,82 @@
+// @rstest-environment jsdom
+import { expect, test, beforeEach, rstest } from '@rstest/core';
+import {
+  rehydrateApiCache,
+  startApiPersistence,
+} from '../src/store/rtkqPersist';
+import { REHYDRATE_API } from '../src/api/rehydrate';
+import {
+  upsertInstance,
+  setActiveInstanceId,
+  instanceStorage,
+} from '../src/config/instances';
+import type { RootState } from '../src/store/index';
+
+beforeEach(() => {
+  localStorage.clear();
+  const inst = upsertInstance('http://test.local');
+  setActiveInstanceId(inst.id);
+});
+
+function apiStateWith(queries: Record<string, unknown>) {
+  return {
+    api: { queries, mutations: {}, provided: {}, subscriptions: {}, config: {} },
+  } as unknown as RootState;
+}
+
+test('rehydrateApiCache dispatches nothing when no cache is stored', () => {
+  const dispatched: unknown[] = [];
+  rehydrateApiCache((a) => dispatched.push(a));
+  expect(dispatched).toHaveLength(0);
+});
+
+test('rehydrateApiCache replays a stored cache as a rehydrate action', () => {
+  instanceStorage.set(
+    'rtkq',
+    JSON.stringify({
+      queries: { 'getLibrary(undefined)': { status: 'fulfilled', data: [1] } },
+      mutations: {},
+    }),
+  );
+  const dispatched: Array<{ type: string; payload: unknown }> = [];
+  rehydrateApiCache((a) =>
+    dispatched.push(a as { type: string; payload: unknown }),
+  );
+  expect(dispatched).toHaveLength(1);
+  expect(dispatched[0].type).toBe(REHYDRATE_API);
+  expect(dispatched[0].payload).toMatchObject({
+    queries: { 'getLibrary(undefined)': { status: 'fulfilled' } },
+  });
+});
+
+test('startApiPersistence saves only fulfilled queries after throttle', () => {
+  rstest.useFakeTimers();
+  let state = apiStateWith({});
+  let listener: (() => void) | null = null;
+  const store = {
+    getState: () => state,
+    subscribe: (l: () => void) => {
+      listener = l;
+      return () => {};
+    },
+  };
+
+  startApiPersistence(store);
+
+  state = apiStateWith({
+    'getAlbums(undefined)': { status: 'fulfilled', data: ['a'] },
+    'getArtists(undefined)': { status: 'pending' },
+    'getTracks(undefined)': { status: 'rejected', error: 'boom' },
+  });
+  listener!();
+
+  // throttled — nothing yet
+  expect(instanceStorage.get('rtkq')).toBeNull();
+
+  rstest.advanceTimersByTime(2000);
+  const saved = JSON.parse(instanceStorage.get('rtkq')!);
+  expect(Object.keys(saved.queries)).toEqual(['getAlbums(undefined)']);
+  expect(saved.mutations).toEqual({});
+
+  rstest.useRealTimers();
+});