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/message/components/ChatScreen/styles.ts

@@ -106,12 +106,12 @@ export function createChatScreenStyles(colors: AppColors, dynamicStyles?: {
     height: 56,
   },
   backButton: {
-    width: 36,
-    height: 36,
+    width: 40,
+    height: 40,
     alignItems: 'center',
     justifyContent: 'center',
-    borderRadius: 10,
-    backgroundColor: 'transparent',
+    marginLeft: -12,
+    padding: 0,
   },
   headerCenter: {
     flex: 1,