feat: RDtop RustDesk headless fallback for Intel iGPU

- Adds VIRTUAL1 as permanent primary output via modesetting driver
- HDMI1 set as clone (--same-as) of VIRTUAL1
- hdmi-fallback.sh script monitors drm connector status
- systemd user unit auto-creates VIRTUAL1 and switches on hotplug loss
- Xorg fallback guarantees 1920x1080 framebuffer without HDMI
- Includes xserver-xorg-video-dummy as backup driver

config/systemd/hdmi-fallback.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=HDMI to VIRTUAL1 Fallback Monitor
+After=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=%h/.local/bin/hdmi-fallback.sh
+Restart=always
+RestartSec=5
+Environment="DISPLAY=:0"
+
+[Install]
+WantedBy=default.target