fix: keep screen signaling WebSocket alive

[rogniceguy-max]

Summary

• keep the screen signaling WebSocket alive after SDP negotiation
• continuously read the socket so Gorilla can process close, ping, and pong control frames
• send protocol-level pings and refresh the read deadline on pong
• handle the existing binary 0x63 key-frame request by calling agent.PLIRequest()

Problem

handleScreenWS returned immediately after sending webrtc_metainfo. That left no reader for the upgraded WebSocket, so control frames and the key-frame request sent by public/static/connect.js were never consumed. Idle signaling connections could also be reaped while the WebRTC peer remained healthy.

Verification

• built the embedded Linux recorder required by the project
• ran go test ./... successfully with Go 1.25.4

[huonwe]

Hi, I'm sorry for the late reply. I was busy over the last few months. I'd like to work on this project again soon, as well as merge your great contribution. Thanks!