Fix WebSocket 'Invalid frame header' error by disabling WebSocket transport

This change resolves the WebSocket connection error that occurs when Cloudflare
or other reverse proxies block WebSocket upgrade attempts.

Changes:
- Frontend: Configure Socket.IO client to use polling-only transport
- Backend: Add documentation comment about transport configuration
- Remove WebSocket URL conversion (no longer needed for polling)

The error occurred because:
1. Socket.IO started with HTTP polling (successful)
2. Attempted to upgrade to WebSocket (blocked by Cloudflare)
3. Browser received invalid/blocked frames causing "Invalid frame header"
4. Eventually fell back to polling (working)

With this fix:
- Socket.IO uses HTTP long-polling exclusively
- No WebSocket upgrade attempts
- No "Invalid frame header" errors
- Connection remains stable through Cloudflare

Polling transport provides equivalent functionality and reliability.

https://claude.ai/code/session_mmQs0

backend/app.py

@@ -22,6 +22,8 @@ app = Flask(__name__)
 CORS(app, resources={r"/*": {"origins": "*"}})
 
 # Initialize SocketIO
+# Note: Frontend uses polling-only transport due to Cloudflare/reverse proxy
+# blocking WebSocket connections. Server supports both transports.
 socketio = SocketIO(
     app,
     cors_allowed_origins="*",

frontend/lib/hooks/useInteractiveTerminal.ts

@@ -114,9 +114,10 @@ export function useInteractiveTerminal({
           (window as any)._debugTerminal = term;
         }
 
-        const wsUrl = API_BASE_URL.replace(/^http/, 'ws');
-        socket = io(`${wsUrl}/terminal`, {
-          transports: ['polling', 'websocket'],
+        // Use polling only - WebSocket is blocked by Cloudflare/reverse proxy
+        // This prevents "Invalid frame header" errors during upgrade attempts
+        socket = io(`${API_BASE_URL}/terminal`, {
+          transports: ['polling'],
           reconnectionDelayMax: 10000,
           timeout: 60000,
           forceNew: true,