refactor(database): migrate to new modular database layer and unify data access

- Remove legacy database.ts, LocalDataSource.ts, and MessageRepository.ts
- Create new src/database/ module with messageRepository, userCacheRepository, conversationRepository, and groupCacheRepository
- Update all consumers to import from @/database instead of services/database
- Add web platform blur handling for modal components to fix focus issues
- Flatten SystemMessageItem and NotificationsScreen styles for consistent design
- Add draggable slider in ChatSettingsScreen and dynamic font size support
- Introduce 9 new chat color themes
- Add profile screens for about, terms, and privacy policy with navigation routes
- Add policy links to login and registration screens
- Fix post share URL format from /posts/ to /post/

src/screens/profile/index.ts

@@ -14,6 +14,9 @@ export { default as FollowListScreen } from './FollowListScreen';
 export { NotificationSettingsScreen } from './NotificationSettingsScreen';
 export { BlockedUsersScreen } from './BlockedUsersScreen';
 export { AccountSecurityScreen } from './AccountSecurityScreen';
+export { AboutScreen } from './AboutScreen';
+export { TermsOfServiceScreen } from './TermsOfServiceScreen';
+export { PrivacyPolicyScreen } from './PrivacyPolicyScreen';
 
 // 导出 Hook 供需要自定义的场景使用
 export { useUserProfile, createSharedProfileStyles, TABS, TAB_ICONS } from './useUserProfile';