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/components/ChatScreen/styles.ts

@@ -6,7 +6,7 @@
 import { useMemo } from 'react';
 import { StyleSheet, Dimensions } from 'react-native';
 import { spacing, shadows, useAppColors, type AppColors } from '../../../../theme';
-import { useChatSettingsStore, CHAT_THEMES } from '../../../../stores/chatSettingsStore';
+import { useChatSettingsStore, CHAT_THEMES } from '../../../../stores/settings';
 
 const { width: SCREEN_WIDTH } = Dimensions.get('window');