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/options-core.ts

@@ -642,6 +642,13 @@ export function buildCoreConfigOptionRegistry(
       defaultValue: defaultConfig.shortcuts.openSessionHelp,
       description: 'Accelerator that opens the session help / keybinding cheatsheet.',
     },
+    {
+      path: 'shortcuts.openSubtitleSelection',
+      kind: 'string',
+      defaultValue: defaultConfig.shortcuts.openSubtitleSelection,
+      description:
+        'Open subtitle selection when enabled. Use g-s to press g then s. Set null to unbind.',
+    },
     {
       path: 'shortcuts.openSubtitleGeneration',
       kind: 'string',