fix: IPC byte order, globales GTK-Theme, Session-Vorauswahl

- ipc.py: !I (Big-Endian) → =I (Native Byte Order) für greetd-Protokoll
- Per-User GTK-Theme entfernt, stattdessen globales Theme aus moongreet.toml
- Last-Session pro User in /var/cache/moongreet/last-session/ speichern/laden
- PKGBUILD und install-Hook für last-session-Cache erweitert

config/moongreet.toml

@@ -4,3 +4,5 @@
 [appearance]
 # Absolute path to wallpaper image
 background = "/usr/share/backgrounds/wallpaper.jpg"
+# GTK theme for the greeter UI
+gtk-theme = "Catppuccin-Mocha-Standard-Blue-Dark"