Fix remote input: give the compositor a libinput backend

The stream rendered but ignored keyboard and mouse. Sunshine injects input
through uinput exclusively — it has no support for the Wayland virtual
keyboard/pointer protocols — so the compositor has to have a libinput
backend to see those devices. WLR_BACKENDS=headless creates only the
headless backend, so sway had zero input devices against the kernel's 24.

libinput needs a seat, and a systemd --user service belongs to no logind
session, so libseat needs seatd. seatd is now enabled on this host.

Its socket defaults to group `seat`, which does not help here: the user
manager's group list is fixed when it starts, Linger=yes means that is at
boot, and so adding connor to `seat` would not take effect until a reboot —
re-logging in changes nothing. The drop-in points seatd at `video`, a group
connor already holds for GPU access, which makes this deterministic instead
of dependent on group-propagation timing.

sway now reports 7 input devices including libvirtualhid Mouse and
Keyboard, which are Sunshine's.

hosts/mainframe/etc/ is a new tree for system-level config; install flags
it the same way it flags sbin.

hosts/mainframe/etc/systemd/system/seatd.service.d/override.conf

@@ -0,0 +1,11 @@
+# seatd's socket defaults to group `seat`. The compositor runs as a systemd
+# --user service, and that manager's group list is fixed at the moment it
+# starts — with Linger=yes it starts at boot, so adding connor to `seat` does
+# not reach it until a reboot, and re-logging in changes nothing.
+#
+# `video` is a group connor already holds (it is required for GPU access
+# anyway), so pointing seatd at it makes seat access deterministic rather than
+# dependent on group-propagation timing.
+[Service]
+ExecStart=
+ExecStart=/usr/bin/seatd -g video