fix: power buttons via systemctl, single greeter window (v0.8.7)

Reboot/shutdown buttons always failed: power.rs called `loginctl
reboot|poweroff`, but loginctl has no such verbs (systemd 260) — those
belong to systemctl. moonlock/moonset already used systemctl; moongreet
was the outlier. Switch to `systemctl --no-ask-password reboot|poweroff`.

The multi-monitor greeter gave Exclusive keyboard only to the first
monitor's window, so a user focused on any other output could not type
the password. Drop the per-monitor loop + hotplug; create one window on
the focused output (no set_monitor) with Exclusive keyboard.

Polkit rule kept as a harmless safety net (it was never the blocker;
CanReboot returns yes). The missing journal errors were not a logging
bug — they were lost to a hard power-cut before journald synced.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.8.6"
+version = "0.8.7"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"