feat(ui): unify design system to Twitter/X style across screens and improve message handling

Redesign multiple screens and components from card-based responsive design to Twitter/X flat design:
- UserProfileHeader: adopt Twitter/X style with larger icons, simplified buttons, and block functionality
- AppBackButton: update to chevron-left icon with larger size, remove background styling
- Update SettingsScreen, EditProfileScreen, and UserProfileScreen with flat layout approach
- ConversationListRow: convert from bordered cards to flat list rows with updated typography

Improve message system with WebSocket notification handling:
- Add real-time system unread count updates via WebSocket
- Track lastSystemMessageAt for accurate system message timestamps
- Add notification deduplication and read-status filtering
- Combine system and conversation unread counts in background sync
- Clear system notifications from notification center on mark as read

src/screens/home/PostDetailScreen.tsx

@@ -1929,10 +1929,8 @@ function createPostDetailStyles(colors: AppColors) {
     marginRight: spacing.sm,
   },
   headerBackButton: {
-    paddingHorizontal: spacing.xs,
-    paddingVertical: spacing.xs,
-    marginLeft: spacing.xs,
-    marginRight: spacing.sm,
+    marginLeft: 0,
+    marginRight: 0,
   },
   // 帖子容器
   postContainer: {