fix: add pong/timeout detection to prevent silent connection hangs

- Backend now responds to __ping__ with __pong__
- Frontend detects missing pong within 5s and closes connection
- This triggers proper reconnect/auth recovery instead of hanging
- Fixes 'keepalive ping timeout' issue where connection appears alive but is dead

dashboard/backend/routers/terminal.py

@@ -180,6 +180,7 @@ async def ws_endpoint(websocket: WebSocket, host: str = Query("nas"), token: str
                         process.stdin.write(data)
                 elif "text" in msg and msg["text"]:
                     if msg["text"] == "__ping__":
+                        await websocket.send_text("__pong__")
                         continue
                     process.stdin.write(msg["text"].encode())
         except Exception as e: