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/locales/en/player.ts

@@ -51,6 +51,7 @@ export const player = {
   fsLyricsToggle: 'Lyrics in fullscreen',
   lyricsLoading: 'Loading lyrics…',
   lyricsNotFound: 'No lyrics found for this track',
+  lyricsNoSources: 'No lyrics sources selected — enable some in Settings → Lyrics',
   lyricsSourceServer: 'Source: Server',
   lyricsSourceLrclib: 'Source: LRCLIB',
   lyricsSourceNetease: 'Source: Netease',

src/locales/en/settings.ts

@@ -220,10 +220,10 @@ export const settings = {
   lyricsSourceServer: 'Server',
   lyricsSourceLrclib: 'LRCLIB',
   lyricsSourceNetease: 'Netease Cloud Music',
-  lyricsModeStandard: 'Standard',
-  lyricsModeStandardDesc: 'Three sources in a freely orderable list: server tags, LRCLIB, Netease.',
-  lyricsModeLyricsplus: 'YouLyPlus (Karaoke)',
-  lyricsModeLyricsplusDesc: 'Word-by-word sync sourced from Apple Music, Spotify, Musixmatch & QQ (community backend). Silently falls back to the standard sources when nothing is found.',
+  lyricsYouLyPlus: 'YouLyPlus (Karaoke)',
+  lyricsYouLyPlusDesc: 'Word-by-word sync from a community backend (Apple Music, Spotify, Musixmatch, QQ). Tried first; the enabled sources below act as fallback.',
+  lyricsSourcesFallbackHint: 'Used as fallback when YouLyPlus finds nothing. Drag to reorder.',
+  lyricsSourcesPrimaryHint: 'Tried in order. Drag to reorder. All off = lyrics disabled.',
   lyricsStaticOnly: 'Show lyrics as static text only',
   lyricsStaticOnlyDesc: 'Render synced lyrics without auto-scroll and without word highlighting.',
   downloadsTitle: 'ZIP Export & Archiving',