Make the headless session actually usable over the stream

Two reasons both Sunshine apps streamed a blank screen.

Steam is an X client, and sway starts Xwayland lazily. DISPLAY therefore
did not exist at the moment the config imported the environment for
Sunshine, so Steam came up with "Unable to open a connection to X" and
rendered nothing. `xwayland force` starts it at compositor launch instead,
and DISPLAY joins the import.

The Desktop app was not broken at all — the desktop was genuinely empty.
swaybg was painting #101920 on an output with no windows and no bar, which
over a stream is indistinguishable from a failed capture. There are now
keybinds for a terminal and a launcher, and kitty starts with the session
so a working capture looks obviously working.

Verified: Xwayland on :0, DISPLAY exported, xdpyinfo and zenity connect,
and Steam Big Picture Mode renders as a window on HEADLESS-1.

hosts/mainframe/sway/headless.conf

@@ -8,15 +8,28 @@
 output HEADLESS-1 mode 1920x1080@60Hz position 0,0
 output HEADLESS-1 background #101920 solid_color
 
-# Sunshine is a systemd user unit, so it needs WAYLAND_DISPLAY in the user
+# Start Xwayland eagerly rather than on first X client. Steam is an X app, and
-# manager's environment. Import it before starting Sunshine, not after.
+# with the default lazy start DISPLAY does not exist yet when the line below
-exec systemctl --user import-environment WAYLAND_DISPLAY XDG_CURRENT_DESKTOP
+# imports the environment — so Steam launched by Sunshine got
+# "Unable to open a connection to X" and showed a blank stream.
+xwayland force
+
+# Sunshine is a systemd user unit and inherits nothing from this session, so
+# hand it the display variables before starting it.
+exec systemctl --user import-environment WAYLAND_DISPLAY DISPLAY XDG_CURRENT_DESKTOP
 exec systemctl --user start sunshine.service
 
-# Nothing is attached to this session locally, so keybinds are only useful
+# Input arrives from the Moonlight client, so keybinds are worth having even
-# through a Moonlight client. Keep the set minimal.
+# though nobody is sitting at this machine. Without them the Desktop app
+# streams an empty screen with no way to open anything.
 set $mod Mod4
+bindsym $mod+Return exec kitty
+bindsym $mod+d      exec wofi --show drun
 bindsym $mod+Shift+q kill
 bindsym $mod+Shift+e exec swaymsg exit
 
+# Something visible on login, so a working capture is obviously working rather
+# than indistinguishable from a broken one.
+exec kitty
+
 # No bar, no idle daemon, no lock screen — there is no one sitting here.