feat: add optional background blur via image crate

Gaussian blur applied at texture load time when `background-blur` is
set in the [appearance] section of moongreet.toml. Blur runs once,
result is shared across monitors.

src/main.rs

@@ -55,7 +55,7 @@ fn activate(app: &gtk::Application) {
     log::debug!("Background path: {}", bg_path.display());
 
     // Load background texture once — shared across all windows
-    let bg_texture = greeter::load_background_texture(&bg_path);
+    let bg_texture = greeter::load_background_texture(&bg_path, config.background_blur);
     if bg_texture.is_none() {
         log::error!("Failed to load background texture — greeter will start without wallpaper");
     }