feat(overlay): add subtitle selection modal and Jellyfin 12 fixes

- Add an optional subtitle selection modal for primary/secondary mpv tracks (subtitleSelection.enabled, g-s sequence shortcut) with key-sequence conflict handling
- Authenticate Jellyfin URLs with the ApiKey query, answer remote keep-alives, clear now-playing on stop, and restore episode titles in Anki misc info
- Honor the configured mpv executable when Jellyfin starts playback via a shared mpv-process launcher
- Bump electron-builder to 26.16.1
- Condense and reconcile changelog fragments; update config example and docs

src/config/definitions.ts

@@ -58,6 +58,7 @@ const { immersionTracking } = IMMERSION_DEFAULT_CONFIG;
 const { stats } = STATS_DEFAULT_CONFIG;
 
 export const DEFAULT_CONFIG: ResolvedConfig = {
+  subtitleSelection: { enabled: false },
   subtitleGeneration: { ...DEFAULT_SUBTITLE_GENERATION_CONFIG },
   dictionaryBackend,
   hachidori,