refactor(core): distinguish network errors from auth failures and refactor real-time messaging pipeline

- Add isNetworkError() helper and FetchCurrentUserResult discriminated union to prevent logout on network failures
- Refactor authService to return { kind: 'user' } | { kind: 'auth_failed' } | { kind: 'network_error' }
- Update authStore to preserve login state on network errors, only logout on auth failures
- Replace WSMessageHandler with RealtimeIngestionPipeline for improved real-time message handling
- Remove MessageDeduplication service; add store-level idempotent addOrReplaceMessage for message dedup
- Add atomic systemUnreadCount operations to prevent race conditions
- Add SearchHeader component for consistent search UI across screens
- Add 'home' TabBar variant with underline style matching HomeScreen
- Add Jest test infrastructure and exclude tests from tsconfig
- Fix ChatScreen history lock being stuck when scrolling to latest messages
- Remove unused call_participant_joined/left WS event types

src/services/core/index.ts

@@ -1,5 +1,5 @@
 export { api, WS_URL, TOKEN_KEY, REFRESH_TOKEN_KEY } from './api';
-export { ApiError } from './api';
+export { ApiError, isNetworkError } from './api';
 export type { ApiResponse, PaginatedData } from './api';
 
 export { wsService } from './wsService';