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/

app/(app)/(tabs)/profile/_layout.tsx

@@ -36,6 +36,10 @@ export default function ProfileStackLayout() {
       <Stack.Screen name="bookmarks" options={{ title: '收藏' }} />
       <Stack.Screen name="notification-settings" options={{ title: '通知设置' }} />
       <Stack.Screen name="blocked-users" options={{ title: '黑名单' }} />
+      <Stack.Screen name="chat-settings" options={{ title: '聊天设置' }} />
+      <Stack.Screen name="about" options={{ title: '关于我们' }} />
+      <Stack.Screen name="terms" options={{ title: '用户协议' }} />
+      <Stack.Screen name="privacy" options={{ title: '隐私政策' }} />
     </Stack>
   );
 }