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.

src/power.rs

@@ -1,4 +1,4 @@
-// ABOUTME: Power actions — reboot and shutdown via loginctl.
+// ABOUTME: Power actions — reboot and shutdown via systemctl.
 // ABOUTME: Wrappers around system commands for the greeter UI.
 
 use std::fmt;
@@ -99,14 +99,21 @@ fn run_command(action: &'static str, program: &str, args: &[&str]) -> Result<(),
     }
 }
 
-/// Reboot the system via loginctl.
+/// Reboot the system via systemctl.
+///
+/// `--no-ask-password` keeps systemctl from spawning an interactive askpass
+/// agent — the greeter session has none, so without it a denied authorization
+/// would hang instead of failing fast.
 pub fn reboot() -> Result<(), PowerError> {
-    run_command("reboot", "/usr/bin/loginctl", &["reboot"])
+    run_command("reboot", "/usr/bin/systemctl", &["--no-ask-password", "reboot"])
 }
 
-/// Shut down the system via loginctl.
+/// Shut down the system via systemctl.
+///
+/// `--no-ask-password` for the same reason as [`reboot`] — the agent-less
+/// greeter session has nothing to answer an authorization challenge.
 pub fn shutdown() -> Result<(), PowerError> {
-    run_command("shutdown", "/usr/bin/loginctl", &["poweroff"])
+    run_command("shutdown", "/usr/bin/systemctl", &["--no-ask-password", "poweroff"])
 }
 
 #[cfg(test)]