feat: show greeter UI on all monitors, not just one (v0.8.0)

Wayland surfaces belong to exactly one output — mirroring is not an option.
Create one full greeter window per monitor via set_monitor(), with only the
first receiving KeyboardMode::Exclusive. Removes the old wallpaper-only
secondary windows. Matches moonlock's per-monitor pattern.

CLAUDE.md

@@ -47,7 +47,7 @@ cd pkg && makepkg -sf && sudo pacman -U moongreet-git-<version>-x86_64.pkg.tar.z
 - `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, systemd-journal-logger
+- `main.rs` — Entry Point, GTK App, Layer Shell Setup, Multi-Monitor (one greeter window per monitor, first gets keyboard), systemd-journal-logger
 - `resources/style.css` — Catppuccin-inspiriertes Theme
 
 ## Design Decisions