fix: Matrix invitation user ID formatting

Fixed MatrixError: Expected UserID string to start with '@'

The issue was that Matrix user IDs must always start with '@' but the
inviteToRoom method was not ensuring proper formatting.

Changes:
- Added user ID formatting in MatrixService.inviteToRoom()
- Ensures userId starts with '@' before sending to Matrix client
- Added logging to show original vs formatted user ID
- Handles cases where user IDs come without '@' prefix

This fixes invitation failures when user IDs don't have proper Matrix format.

ui/desktop/src/services/MatrixService.ts

@@ -870,7 +870,17 @@ export class MatrixService extends EventEmitter {
       throw new Error('Client not initialized');
     }
 
-    await this.client.invite(roomId, userId);
+    // Ensure userId starts with '@' as required by Matrix
+    const formattedUserId = userId.startsWith('@') ? userId : `@${userId}`;
+    
+    console.log('🔗 Inviting user to room:', {
+      roomId,
+      originalUserId: userId,
+      formattedUserId,
+      needsFormatting: !userId.startsWith('@')
+    });
+
+    await this.client.invite(roomId, formattedUserId);
   }
 
   /**