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>

src/store/selectors/localLibrary.ts

@@ -0,0 +1,97 @@
+/*
+ * Offline library composition. When the active backend is unreachable, a single
+ * `getTracks` query may be `rejected` (or never matched a rehydrated arg), so we
+ * can't rely on it to render the library. Instead we compose the "locally
+ * available" library from *every* fulfilled entry in the RTK Query cache —
+ * last-seen lists rehydrated from localStorage (Tier 2) plus anything fetched
+ * this session. This is read-only derived data, not a server-data slice copy:
+ * it reads straight from the RTKQ cache the architecture already owns.
+ */
+import { createSelector } from '@reduxjs/toolkit';
+import type { RootState } from '../index';
+import type { Album, Artist, PaginatedResponse, Track } from '../../api/types';
+
+interface CacheEntry {
+  status: string;
+  endpointName?: string;
+  data?: unknown;
+}
+
+const selectQueries = (state: RootState): Record<string, unknown> =>
+  state.api.queries;
+
+function fulfilled(queries: Record<string, unknown>): CacheEntry[] {
+  const out: CacheEntry[] = [];
+  for (const entry of Object.values(queries)) {
+    const e = entry as CacheEntry | undefined;
+    if (e && e.status === 'fulfilled' && e.data != null) out.push(e);
+  }
+  return out;
+}
+
+/** Every track known locally, deduped by id (last write wins). */
+export const selectLocalTracks = createSelector(
+  selectQueries,
+  (queries): Track[] => {
+    const byId = new Map<string, Track>();
+    for (const e of fulfilled(queries)) {
+      switch (e.endpointName) {
+        case 'getTracks':
+          for (const t of (e.data as PaginatedResponse<Track>).items)
+            byId.set(t.id, t);
+          break;
+        case 'getAlbumTracks':
+        case 'getArtistTracks':
+          for (const t of e.data as Track[]) byId.set(t.id, t);
+          break;
+        case 'getTrack':
+          byId.set((e.data as Track).id, e.data as Track);
+          break;
+      }
+    }
+    return [...byId.values()];
+  },
+);
+
+/** Every album known locally, deduped by id. */
+export const selectLocalAlbums = createSelector(
+  selectQueries,
+  (queries): Album[] => {
+    const byId = new Map<string, Album>();
+    for (const e of fulfilled(queries)) {
+      switch (e.endpointName) {
+        case 'getAlbums':
+          for (const a of (e.data as PaginatedResponse<Album>).items)
+            byId.set(a.id, a);
+          break;
+        case 'getArtistAlbums':
+          for (const a of e.data as Album[]) byId.set(a.id, a);
+          break;
+        case 'getAlbum':
+          byId.set((e.data as Album).id, e.data as Album);
+          break;
+      }
+    }
+    return [...byId.values()];
+  },
+);
+
+/** Every artist known locally, deduped by id. */
+export const selectLocalArtists = createSelector(
+  selectQueries,
+  (queries): Artist[] => {
+    const byId = new Map<string, Artist>();
+    for (const e of fulfilled(queries)) {
+      switch (e.endpointName) {
+        case 'getArtists':
+          for (const a of (e.data as PaginatedResponse<Artist>).items)
+            byId.set(a.id, a);
+          break;
+        case 'getArtist':
+          byId.set((e.data as Artist).id, e.data as Artist);
+          break;
+      }
+    }
+    return [...byId.values()];
+  },
+);