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.

README.md

@@ -12,7 +12,7 @@ Part of the Moonarch ecosystem.
 - **Last user/session** — Remembered in `/var/cache/moongreet/`
 - **Power actions** — Reboot / Shutdown via `loginctl`
 - **Layer Shell** — Fullscreen via gtk4-layer-shell (TOP layer)
-- **Multi-monitor** — Greeter on primary, wallpaper on all monitors
+- **Multi-monitor** — Full greeter UI on all monitors (keyboard input on first)
 - **GPU blur** — Background blur via GskBlurNode (shared cache across monitors)
 - **i18n** — German and English (auto-detected from system locale)
 - **Faillock warning** — Warns after 2 failed attempts, locked message after 3