feat: Default-Wallpaper-Fallback und neues Wallpaper

Greeter fällt auf mitgeliefertes Package-Wallpaper zurück, wenn kein
Background konfiguriert ist. Wallpaper ersetzt durch Daniel Leone's
"Snowy Mountain" (Unsplash License).

src/moongreet/greeter.py

@@ -25,6 +25,7 @@ LAST_USER_PATH = Path("/var/cache/moongreet/last-user")
 FAILLOCK_MAX_ATTEMPTS = 3
 PACKAGE_DATA = files("moongreet") / "data"
 DEFAULT_AVATAR_PATH = PACKAGE_DATA / "default-avatar.svg"
+DEFAULT_WALLPAPER_PATH = PACKAGE_DATA / "wallpaper.jpg"
 AVATAR_SIZE = 128
 
 
@@ -69,9 +70,12 @@ class GreeterWindow(Gtk.ApplicationWindow):
         self.set_child(overlay)
 
         # Background wallpaper (blurred and darkened)
-        if self._config.background and self._config.background.exists():
+        bg_path = self._config.background
+        if not bg_path or not bg_path.exists():
+            bg_path = Path(str(DEFAULT_WALLPAPER_PATH))
+        if bg_path.exists():
             background = Gtk.Picture()
-            background.set_filename(str(self._config.background))
+            background.set_filename(str(bg_path))
             background.set_content_fit(Gtk.ContentFit.COVER)
             background.set_hexpand(True)
             background.set_vexpand(True)