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/nl.ts

@@ -489,6 +489,11 @@ export const nlTranslation = {
     lyricsServerFirstDesc: 'Controleer eerst door de server geleverde songteksten (ingebedde tags, sidecar-bestanden) vóór LRCLIB. Uitschakelen om LRCLIB eerst te gebruiken.',
     enableNeteaselyrics: 'Netease Cloud Music songteksten',
     enableNeteaselyricsDesc: 'Gebruik Netease Cloud Music als laatste bron wanneer server en LRCLIB niets opleveren. Beste dekking voor Aziatische en internationale muziek.',
+    lyricsSourcesTitle: 'Songtekstbronnen',
+    lyricsSourcesDesc: 'Kies welke bronnen worden geraadpleegd en in welke volgorde. Sleep om te sorteren. Uitgeschakelde bronnen worden overgeslagen.',
+    lyricsSourceServer: 'Server',
+    lyricsSourceLrclib: 'LRCLIB',
+    lyricsSourceNetease: 'Netease Cloud Music',
     downloadsTitle: 'ZIP-export & Archivering',
     downloadsFolderDesc: 'Doelmap voor albums die je als ZIP-bestand naar je computer downloadt.',
     downloadsDefault: 'Standaard downloadmap',
@@ -572,6 +577,8 @@ export const nlTranslation = {
     replayGainMode: 'Modus',
     replayGainTrack: 'Nummer',
     replayGainAlbum: 'Album',
+    replayGainPreGain: 'Pre-Gain (getagde bestanden)',
+    replayGainFallback: 'Terugval (zonder tags / radio)',
     crossfade: 'Overgang',
     crossfadeDesc: 'Fade tussen nummers',
     crossfadeSecs: '{{n}} s',