feat: add optional background blur via image crate

Gaussian blur applied at texture load time when `background_blur` is
set in moonset.toml. Blur runs once, result is shared across monitors.

src/main.rs

@@ -56,7 +56,7 @@ fn activate(app: &gtk::Application) {
     // Resolve wallpaper once, decode texture once, share across all windows
     let config = config::load_config(None);
     let bg_path = config::resolve_background_path(&config);
-    let texture = panel::load_background_texture(&bg_path);
+    let texture = panel::load_background_texture(&bg_path, config.background_blur);
 
     // Panel on focused output (no set_monitor → compositor picks focused)
     let panel = panel::create_panel_window(&texture, app);