[FEATURE] Optional screen / window capture track in WebRTC sessions

[krwg]

Summary

Add opt-in screen share alongside existing audio path using getDisplayMedia with explicit UX surface and hang-up semantics independent of microphone.

Background

LAN classroom / coworking demos parity with mainstream messengers albeit bandwidth heavier.

Scope

• Negotiation path adds video m-line gated by mutual consent handshake (offer flag already partially present — extend thoughtfully).
• UI affordance distinguish share vs webcam future placeholder.
• Windows capture constraints documented; DPI scaling caveats surfaced as known issues readme section.

Acceptance criteria

• Recipient sees shared track in call window scalable surface.
• Stopping share removes sender track without nuking mic session.
• CPU guard (lower max frame rate presets).

Technical notes

Plan for Electron desktopCapturer fallbacks vs pure web path.

Definition of done

Stress test simultaneous audio + moderate motion screen on same machine two-instance setup.