feat(replaygain): add Auto mode that picks track vs album gain from queue context (#242)

Auto mode is the new default for ReplayGain. It picks album gain when an
adjacent queue neighbour shares the same albumId (so "Play All" on an
album, or any contiguous album block, gets album gain), otherwise track
gain. Falls back to track gain when album gain is missing.

- authStore: replayGainMode widened to 'track' | 'album' | 'auto', default 'auto'
- playerStore: new resolveReplayGainDb(track, prev, next, enabled, mode) helper
  replaces five inline ternaries (audio_play hot path, gapless preload,
  cold-resume success + fallback, live update_replay_gain)
- Settings: third "Auto" button + contextual hint when active
- i18n: replayGainAuto + replayGainAutoDesc added to all 8 locales

Co-authored-by: Psychotoxical <dev@psysonic.app>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/store/authStore.ts

@@ -49,7 +49,7 @@ interface AuthState {
   excludeAudiobooks: boolean;
   customGenreBlacklist: string[];
   replayGainEnabled: boolean;
-  replayGainMode: 'track' | 'album';
+  replayGainMode: 'track' | 'album' | 'auto';
   replayGainPreGainDb: number;   // added to RG gain for tagged files (0…+6 dB)
   replayGainFallbackDb: number;  // gain for untagged files / radio (-6…0 dB)
   crossfadeEnabled: boolean;
@@ -200,7 +200,7 @@ interface AuthState {
   setExcludeAudiobooks: (v: boolean) => void;
   setCustomGenreBlacklist: (v: string[]) => void;
   setReplayGainEnabled: (v: boolean) => void;
-  setReplayGainMode: (v: 'track' | 'album') => void;
+  setReplayGainMode: (v: 'track' | 'album' | 'auto') => void;
   setReplayGainPreGainDb: (v: number) => void;
   setReplayGainFallbackDb: (v: number) => void;
   setCrossfadeEnabled: (v: boolean) => void;
@@ -308,7 +308,7 @@ export const useAuthStore = create<AuthState>()(
       excludeAudiobooks: false,
       customGenreBlacklist: [],
       replayGainEnabled: false,
-      replayGainMode: 'track',
+      replayGainMode: 'auto',
       replayGainPreGainDb: 0,
       replayGainFallbackDb: 0,
       crossfadeEnabled: false,