perf: optimize startup by caching icons, texture, and async avatar

- Replace manual icon theme lookup + Pixbuf scaling with native
  GTK4 Image::from_icon_name() (uses internal cache + GPU rendering)
- Decode wallpaper texture once and share across all windows
  instead of N+1 separate JPEG decodes
- Load file-based avatars asynchronously via gio::spawn_blocking
  to avoid blocking the UI thread

src/main.rs

@@ -51,12 +51,13 @@ fn activate(app: &gtk::Application) {
 
     load_css(&display);
 
-    // Resolve wallpaper once, share across all windows
+    // 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);
 
     // Panel on focused output (no set_monitor → compositor picks focused)
-    let panel = panel::create_panel_window(&bg_path, app);
+    let panel = panel::create_panel_window(&texture, app);
     setup_layer_shell(&panel, true, gtk4_layer_shell::Layer::Overlay);
     panel.present();
 
@@ -64,7 +65,7 @@ fn activate(app: &gtk::Application) {
     let monitors = display.monitors();
     for i in 0..monitors.n_items() {
         if let Some(monitor) = monitors.item(i).and_then(|obj| obj.downcast::<gdk::Monitor>().ok()) {
-            let wallpaper = panel::create_wallpaper_window(&bg_path, app);
+            let wallpaper = panel::create_wallpaper_window(&texture, app);
             setup_layer_shell(&wallpaper, false, gtk4_layer_shell::Layer::Top);
             wallpaper.set_monitor(Some(&monitor));
             wallpaper.present();