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/auth/RegisterScreen.tsx

@@ -309,10 +309,10 @@ function createRegisterStyles(colors: AppColors) {
     backIconButton: {
       width: 40,
       height: 40,
-      borderRadius: 20,
-      backgroundColor: colors.background.default,
       alignItems: 'center',
       justifyContent: 'center',
+      marginLeft: -12,
+      padding: 0,
     },
     headerTitle: {
       fontSize: 18,