build: make the receiver service headless-console ready

The receiver does not need a window manager: SDL3's KMSDRM backend
renders straight to the kernel display pipeline, which is the right
setup for a small-board appliance. But a systemd system service has
no controlling terminal, and SDL's KMSDRM backend expects one for
its VT handling — so the headless path would have failed at boot.
Give the unit StandardInput=tty with TTYPath=/dev/tty1 (harmless in
desktop mode), and document the headless recipe in the unit, the
install script output, and the RUNBOOK: Raspberry Pi OS Lite with
Console Autologin, no desktop enabled (a compositor would hold the
DRM master), and consoleblank=0.

scripts/install-receiver.sh

@@ -138,13 +138,17 @@ The receiver is installed and starts automatically at boot.
     sudo systemctl disable --now screencast-receiver   # turn autostart off
 
 Notes for small boards (e.g. Pi Zero 2 W):
+  - No window manager is required: the receiver renders directly to the
+    kernel display pipeline (SDL KMSDRM). Use Raspberry Pi OS Lite with
+    Console Autologin (raspi-config) and no desktop enabled — a running
+    compositor would own the display and the receiver could not start.
+  - Disable console blanking so the picture never goes dark: add
+    consoleblank=0 to /boot/firmware/cmdline.txt and reboot.
   - Decoding is software H.264; expect smooth playback for small streams
     and a lower frame rate at high resolutions.
   - Stop the receiver via systemctl (never kill -9; stale mDNS records
     would break discovery for ~75 minutes).
-  - The service runs on the headless console via KMSDRM. On a desktop
-    session instead, edit /etc/systemd/system/screencast-receiver.service:
-    uncomment the Environment lines and point XDG_RUNTIME_DIR at the
-    session owner's /run/user/<uid> (see the comments in that file).
+  - On a desktop session instead, edit the installed service file and
+    uncomment the Environment lines for XDG_RUNTIME_DIR/WAYLAND_DISPLAY.
   - avahi-daemon must stay enabled; it is what senders discover.
 EOF