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/home/PostDetailScreen.tsx

@@ -536,7 +536,7 @@ export const PostDetailScreen: React.FC = () => {
     } catch (error) {
       console.error('上报分享次数失败:', error);
     }
-    const postUrl = `https://browser.littlelan.cn/posts/${encodeURIComponent(post.id)}`;
+    const postUrl = `https://browser.littlelan.cn/post/${encodeURIComponent(post.id)}`;
     Clipboard.setString(postUrl);
     Alert.alert('已复制', '帖子链接已复制到剪贴板');
   }, [post?.id]);