feat(settings): wire Playback + Scrobbling panels to the backend

Adds src/api/endpoints/settings.ts (getSettings/updateSettings,
getScrobbling/updateScrobbling) plus AppSettings/ScrobblingConfig types
and a Settings cache tag. Replaces the "coming soon" scaffolds:

- Playback: stream-quality preference (SegmentedControl), server-synced.
- Scrobbling: enable + provider (Last.fm/ListenBrainz) + username +
  write-only session key, with a linked/not-linked badge and save states.

Built with modern-sk (Window/SegmentedControl/TextField/Button/Callout/
Badge) + the shared Loading/Error states; en + ru strings added. Server
state flows through RTK Query hooks only (no fetch in components).

Also drops a pre-existing unused TextField import in LibraryPage that was
failing `tsc --noEmit`.

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

src/api/types.ts

@@ -258,3 +258,24 @@ export interface MetadataEdit {
   genre?: string;
   trackNumber?: number;
 }
+
+// -- user settings (`/settings`) ---------------------------------------------
+export type ThemePref = 'system' | 'light' | 'dark';
+export type StreamQuality = 'original' | 'high' | 'medium' | 'low';
+
+/** General per-user preferences from `GET /settings`. */
+export interface AppSettings {
+  theme: ThemePref;
+  streamQuality: StreamQuality;
+}
+
+export type ScrobbleProvider = 'lastfm' | 'listenbrainz';
+
+/** Scrobbling config from `GET /settings/scrobbling`. The session key is
+ * write-only server-side, so it never appears here — only `configured`. */
+export interface ScrobblingConfig {
+  enabled: boolean;
+  provider: ScrobbleProvider | null;
+  username: string | null;
+  configured: boolean;
+}