refactor(core): introduce EventBus and refactor store infrastructure

- Add EventBus for decoupled event-driven communication between services
- Add EventSubscriber component for centralized event handling
- Add requestDedupe utility for shared request deduplication
- Refactor api/wsService to use eventBus instead of direct router navigation
- Extract MessageMapper.toCachedMessages for consistent message mapping
- Remove deprecated BaseManager and CacheBus classes
- Improve @mention rendering with memberMap support in segment rendering
- Update hooks to use useRef instead of useState for fetch tracking

src/core/events/index.ts

@@ -0,0 +1,2 @@
+export { eventBus, type AppEvent } from './EventBus';
+export { useEventBus } from './useEventBus';