feat(library): render from locally-cached data when offline

The Library showed a blocking error with the backend unreachable. Now it
composes a read-only library from everything already in the RTK Query
cache (Tier-2 rehydrated last-seen data + anything fetched this session),
so it keeps rendering offline instead of erroring.

- selectors: `selectLocalTracks/Albums/Artists` — memoized, union + dedupe
  across getTracks/getAlbums/getArtists, the per-album/artist list
  endpoints, and single-entity fetches; skips pending/rejected entries
- LibraryPage: when offline, fall back to the composed lists (live data
  still wins online), filter client-side for search, show an offline
  banner, and never show the retry-only ErrorState
- i18n: `library.offline.*` (en + ru)
- test: selector composition / dedup / status filtering (3 cases)

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

tests/localLibrary.test.ts

@@ -0,0 +1,87 @@
+import { expect, test } from '@rstest/core';
+import {
+  selectLocalTracks,
+  selectLocalAlbums,
+  selectLocalArtists,
+} from '../src/store/selectors/localLibrary';
+import type { RootState } from '../src/store/index';
+
+function stateWith(queries: Record<string, unknown>): RootState {
+  return { api: { queries } } as unknown as RootState;
+}
+
+const track = (id: string, over: Record<string, unknown> = {}) => ({
+  id,
+  title: `Track ${id}`,
+  artistName: 'A',
+  albumTitle: 'Alb',
+  ...over,
+});
+
+test('selectLocalTracks unions getTracks pages, list endpoints and single tracks', () => {
+  const state = stateWith({
+    'getTracks(undefined)': {
+      status: 'fulfilled',
+      endpointName: 'getTracks',
+      data: { items: [track('1'), track('2')], total: 2 },
+    },
+    'getArtistTracks("x")': {
+      status: 'fulfilled',
+      endpointName: 'getArtistTracks',
+      data: [track('2'), track('3')], // 2 is a dupe
+    },
+    'getTrack("4")': {
+      status: 'fulfilled',
+      endpointName: 'getTrack',
+      data: track('4'),
+    },
+  });
+
+  const ids = selectLocalTracks(state)
+    .map((t) => t.id)
+    .sort();
+  expect(ids).toEqual(['1', '2', '3', '4']);
+});
+
+test('selectLocalTracks ignores pending/rejected and null-data entries', () => {
+  const state = stateWith({
+    'getTracks(a)': {
+      status: 'rejected',
+      endpointName: 'getTracks',
+      data: undefined,
+    },
+    'getTracks(b)': { status: 'pending', endpointName: 'getTracks' },
+    'getTracks(c)': {
+      status: 'fulfilled',
+      endpointName: 'getTracks',
+      data: { items: [track('9')], total: 1 },
+    },
+  });
+  expect(selectLocalTracks(state).map((t) => t.id)).toEqual(['9']);
+});
+
+test('selectLocalAlbums and selectLocalArtists compose and dedupe', () => {
+  const state = stateWith({
+    'getAlbums(undefined)': {
+      status: 'fulfilled',
+      endpointName: 'getAlbums',
+      data: { items: [{ id: 'al1' }, { id: 'al2' }], total: 2 },
+    },
+    'getArtistAlbums("x")': {
+      status: 'fulfilled',
+      endpointName: 'getArtistAlbums',
+      data: [{ id: 'al2' }], // dupe
+    },
+    'getArtists(undefined)': {
+      status: 'fulfilled',
+      endpointName: 'getArtists',
+      data: { items: [{ id: 'ar1' }], total: 1 },
+    },
+  });
+  expect(
+    selectLocalAlbums(state)
+      .map((a) => a.id)
+      .sort(),
+  ).toEqual(['al1', 'al2']);
+  expect(selectLocalArtists(state).map((a) => a.id)).toEqual(['ar1']);
+});