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.

CLAUDE.md

@@ -40,12 +40,12 @@ cd pkg && makepkg -sf && sudo pacman -U moongreet-git-<version>-x86_64.pkg.tar.z
 - `ipc.rs` — greetd Socket-Kommunikation (4-byte LE header + JSON)
 - `users.rs` — Benutzer aus /etc/passwd, Avatare (AccountsService + ~/.face), Symlink-Rejection
 - `sessions.rs` — Wayland/X11 Sessions aus .desktop Files
-- `power.rs` — Reboot/Shutdown via loginctl
+- `power.rs` — Reboot/Shutdown via systemctl (`--no-ask-password`)
 - `i18n.rs` — Locale-Erkennung (LANG / /etc/locale.conf) und String-Tabellen (DE/EN), alle UI- und Login-Fehlermeldungen
 - `fingerprint.rs` — fprintd D-Bus Probe (gio::DBusProxy) — Geräteerkennung und Enrollment-Check für UI-Feedback
 - `config.rs` — TOML-Config ([appearance] background, gtk-theme, fingerprint-enabled) + Wallpaper-Fallback + Blur-Validierung (finite, clamp 0–200)
 - `greeter.rs` — GTK4 UI (Overlay-Layout), Login-Flow via greetd IPC (Multi-Stage-Auth für fprintd), Faillock-Warnung, Avatar-Cache, Last-User/Last-Session Persistence (0o700 Dirs, 0o600 Files)
-- `main.rs` — Entry Point, GTK App, Layer Shell Setup, Multi-Monitor mit Hotplug via `items-changed` auf Monitor-ListModel (one greeter window per monitor, first gets keyboard), systemd-journal-logger
+- `main.rs` — Entry Point, GTK App, Layer Shell Setup, ein Greeter-Fenster auf dem fokussierten Output (kein `set_monitor`), `KeyboardMode::Exclusive`, systemd-journal-logger
 - `resources/style.css` — Catppuccin-inspiriertes Theme
 
 ## Design Decisions