Fix agent crash on missing DISPLAY and relay disconnect error

- Auto-detect DISPLAY on Linux by scanning /tmp/.X11-unix/ sockets,
  falling back to 'w' output, then :0 — runs before mss/pynput import
- ScreenCapture no longer raises on init failure; agent stays connected
  and notifies the viewer with an error message if capture unavailable
- stream_frames skips None frames instead of crashing the WebSocket
- Relay: check for websocket.disconnect message type to avoid
  'Cannot call receive once a disconnect message has been received'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

relay/main.py

@@ -113,6 +113,10 @@ async def agent_endpoint(
         while True:
             msg = await websocket.receive()
 
+            # Client sent a close frame — exit cleanly
+            if msg.get("type") == "websocket.disconnect":
+                break
+
             if "bytes" in msg and msg["bytes"]:
                 # Binary = JPEG frame → forward to all viewers watching this machine
                 frame_data = msg["bytes"]