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/profile/UserScreen.tsx

@@ -7,7 +7,7 @@
 import React from 'react';
 import { useLocalSearchParams } from 'expo-router';
 import UserProfileScreen from './UserProfileScreen';
-import { useCurrentUser } from '../../stores/authStore';
+import { useCurrentUser } from '../../stores/auth';
 
 export const UserScreen: React.FC = () => {
   const { userId: userIdParam } = useLocalSearchParams<{ userId?: string }>();