feat(lyrics): make lyrics fully disablable (independent YouLyPlus toggle) (#855)

* feat(lyrics): independent YouLyPlus toggle + all-sources-off state

Replace the binary lyricsMode ('standard' | 'lyricsplus') with an
independent youLyPlusEnabled flag so YouLyPlus and the standard sources
are no longer mutually exclusive — turning one off no longer forces the
other on. YouLyPlus (when on) is tried first with the enabled sources as
fallback; off uses only the enabled sources. When YouLyPlus is off and no
source is enabled, useLyrics fetches nothing (issue #810).

Fresh installs ship with every source off; the rehydrate migration only
restores the old on-by-default set for genuine upgrades, not new installs.

* feat(lyrics): YouLyPlus toggle UI + queue 'no sources' hint

Settings: single YouLyPlus toggle replacing the two mutually exclusive
mode switches; the source list is always visible with a context hint
(fallback vs primary). Queue lyric tab shows a hint when no source is
active. en + de strings; other locales fall back to en.

* docs(changelog): lyrics fully disablable (#855)

src/store/authStoreDefaults.ts

@@ -28,9 +28,12 @@ export const DEFAULT_TRACK_PREVIEW_LOCATIONS: TrackPreviewLocations = {
   randomMix: true,
 };
 
+// Fresh installs ship with every lyrics source off (issue #810 — users who
+// don't want lyrics get none until they opt in). Existing users keep their
+// persisted `lyricsSources`; the rehydrate migration preserves them.
 export const DEFAULT_LYRICS_SOURCES: LyricsSourceConfig[] = [
-  { id: 'server',  enabled: true  },
-  { id: 'lrclib',  enabled: true  },
+  { id: 'server',  enabled: false },
+  { id: 'lrclib',  enabled: false },
   { id: 'netease', enabled: false },
 ];