feat(09-02): add frontend keyboard shortcut service, store, and controller

- Create keyboard-shortcut-service.ts singleton with keydown listener
- Shadow DOM active element resolution for scope detection
- Text input suppression (only Escape passes through)
- Scope resolution: text-input > panel-specific > global
- Action dispatch to player/queue stores and Wails bindings
- Create shortcuts-store.ts with Wails persistence and event sync
- Create shortcuts-controller.ts for Lit component integration
- Export store and controller from store/index.ts
- Replace hardcoded Ctrl+F handler in index.ts with service
- Initialize service via import in frontend/index.ts

frontend/src/store/index.ts

@@ -9,3 +9,6 @@ export type { ThemeState, BackgroundShade } from './theme-store';
 export { ThemeController } from './controllers/theme-controller';
 export { searchStore } from './search-store';
 export { SearchController } from './controllers/search-controller';
+export { shortcutsStore } from './shortcuts-store';
+export type { ShortcutsState } from './shortcuts-store';
+export { ShortcutsController } from './controllers/shortcuts-controller';