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/GroupRequestDetailScreen.tsx

@@ -7,10 +7,10 @@ import { SafeAreaView } from 'react-native-safe-area-context';
 import { spacing, borderRadius, shadows, useAppColors, type AppColors } from '../../theme';
 import { Avatar, Text } from '../../components/common';
 import * as hrefs from '../../navigation/hrefs';
-import { routePayloadCache } from '../../stores/routePayloadCache';
+import { routePayloadCache } from '../../stores';
 import { groupService } from '@/services/message';
-import { groupManager } from '../../stores/groupManager';
-import { userManager } from '../../stores/userManager';
+import { groupManager } from '../../stores/group';
+import { userManager } from '../../stores/user';
 import { GroupInfoSummaryCard, DecisionFooter } from './components/GroupRequestShared';
 
 const GroupRequestDetailScreen: React.FC = () => {