feat(overlay): add optional subtitle selection modal

- Add primary and secondary mpv track selection from the overlay
- Support configurable sequence shortcuts with conflict detection and hot reload
- Document settings, shortcuts, and generated config defaults

src/main/runtime/global-shortcuts.test.ts

@@ -24,6 +24,7 @@ function createShortcuts(): ConfiguredShortcuts {
     openRuntimeOptions: null,
     openJimaku: null,
     openTsukihime: null,
+    openSubtitleSelection: null,
     openSubtitleGeneration: null,
     openSessionHelp: null,
     openControllerSelect: null,