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/template-sections.ts

@@ -17,6 +17,12 @@ const CORE_TEMPLATE_SECTIONS: ConfigTemplateSection[] = [
     notes: ['Used only while Hachidori is linked to an external host.'],
     key: 'hachidori',
   },
+  {
+    title: 'Subtitle Selection',
+    description: ['Select primary and secondary mpv subtitle tracks from the overlay.'],
+    notes: ['Hot-reload: enabling or disabling updates the session shortcut immediately.'],
+    key: 'subtitleSelection',
+  },
   {
     title: 'Japanese Subtitle Generation',
     description: [