refactor(stores): reorganize flat store files into modular directory structure

Migrate from flat file organization to modular directory structure under src/stores/:

- auth/: authStore, registerStore, verificationStore, sessionStore
- call/: callStore
- settings/: chatSettingsStore, themeStore
- ui/: homeTabBarVisibilityStore, homeTabPressStore
- utils/: routePayloadCache
- group/sources.ts, group/profileResolver.ts
- message/sources.ts
- post/sources.ts

Update all import paths across components and screens to use new module paths. Maintain backward compatibility through deprecated re-export files for gradual migration.

src/screens/message/NotificationsScreen.tsx

@@ -37,7 +37,7 @@ import { Text, ResponsiveContainer } from '../../components/common';
 import { useCursorPagination } from '../../hooks/useCursorPagination';
 import * as hrefs from '../../navigation/hrefs';
 import { useMessageManagerSystemUnreadCount } from '../../stores';
-import { messageManager } from '../../stores/messageManager';
+import { messageManager } from '../../stores/message';
 
 const MESSAGE_TYPES = [
   { key: 'all', title: '全部' },