feat(lyrics): configurable sources with drag-to-reorder + feat(audio): ReplayGain pre-gain & fallback

Lyrics Sources:
- Replace lyricsServerFirst + enableNeteaselyrics toggles with a new
  Settings section — three sources (Server, LRCLIB, Netease) each
  individually toggled and drag-reorderable via psy-drop DnD
- useLyrics.ts iterates sources in user-defined order, skipping disabled
  ones; embedded SYLT from local files still wins unconditionally
- onRehydrateStorage migration from legacy fields on first load

ReplayGain Pre-Gain:
- New authStore fields: replayGainPreGainDb (0…+6 dB) and
  replayGainFallbackDb (-6…0 dB, untagged / radio)
- audio.rs compute_gain applies pre_gain_db and fallback_db
- Settings sliders under ReplayGain mode selector
- Radio HTML5 volume scaled by fallbackDb factor on playRadio

i18n(ru): incorporate PR #148 translation improvements (kilyabin)
All 7 locales updated for new keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/en.ts

@@ -492,6 +492,11 @@ export const enTranslation = {
     lyricsServerFirstDesc: 'Check server-provided lyrics (embedded tags, sidecar files) before querying LRCLIB. Disable to use LRCLIB first.',
     enableNeteaselyrics: 'Netease Cloud Music lyrics',
     enableNeteaselyricsDesc: 'Use Netease Cloud Music as a last-resort lyrics source when server and LRCLIB both return nothing. Best coverage for Asian and international music.',
+    lyricsSourcesTitle: 'Lyrics Sources',
+    lyricsSourcesDesc: 'Choose which sources to query for lyrics and in what order. Drag to reorder. Disabled sources are skipped entirely.',
+    lyricsSourceServer: 'Server',
+    lyricsSourceLrclib: 'LRCLIB',
+    lyricsSourceNetease: 'Netease Cloud Music',
     downloadsTitle: 'ZIP Export & Archiving',
     downloadsFolderDesc: 'Destination folder for albums you download as a ZIP file to your computer.',
     downloadsDefault: 'Default Downloads Folder',
@@ -575,6 +580,8 @@ export const enTranslation = {
     replayGainMode: 'Mode',
     replayGainTrack: 'Track',
     replayGainAlbum: 'Album',
+    replayGainPreGain: 'Pre-Gain (tagged files)',
+    replayGainFallback: 'Fallback (untagged / radio)',
     crossfade: 'Crossfade',
     crossfadeDesc: 'Fade between tracks',
     crossfadeSecs: '{{n}} s',