Rust kiosk:
- web cells now use webkit6 WebView (load_uri)
- html cells use WebView.load_html (full HTML rendering)
- query_displays() reads /sys/class/drm/ for connected HDMI/DP outputs
- Heartbeat reports display geometry every 60s
Server:
- /api/kiosk/heartbeat accepts displays array
- Syncs kiosk-reported displays to display records
- Updates dimensions when changed, creates new displays for new ports