fix(ui): avoid Vite HMR websocket collision

[LiuShenXi]

Summary

• Configure Vite HMR to use a dedicated /__vite_hmr path.
• Avoid collisions with PilotDeck's chat WebSocket proxy on /ws during local dev.

Problem

In local dev mode, the app chat socket also uses /ws. When Vite HMR traffic reaches the PilotDeck chat WebSocket handler, the server can receive a plain ping frame and try to parse it as JSON, producing an error like:

[ERROR] Chat WebSocket error: Unexpected token 'p', "ping" is not valid JSON

After that, the browser UI can remain stuck in Sending... / Thinking even though the prompt never reaches the model provider.

Verification

• pnpm --filter pilotdeck-ui build
• Manually reproduced the local chat flow after isolating HMR from /ws.