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/components/call/CallScreen.tsx

@@ -8,7 +8,7 @@ import {
  StatusBar,
 } from 'react-native';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
-import { callStore } from '../../stores/callStore';
+import { callStore } from '../../stores/call';
 import { RTCView } from 'react-native-webrtc';
 
 const formatDuration = (seconds: number): string => {