feat: activate fade-in animation for panel and wallpaper windows

The Rust code already adds a "visible" CSS class on map, but the
stylesheet had no corresponding opacity transition. Add 250ms ease-in
fade via GPU-accelerated CSS opacity to eliminate the visual pop-in.

resources/style.css

@@ -6,11 +6,23 @@ window.panel {
     background-color: @theme_bg_color;
     background-size: cover;
     background-position: center;
+    opacity: 0;
+    transition: opacity 250ms ease-in;
+}
+
+window.panel.visible {
+    opacity: 1;
 }
 
 /* Wallpaper-only window for secondary monitors */
 window.wallpaper {
     background-color: @theme_bg_color;
+    opacity: 0;
+    transition: opacity 250ms ease-in;
+}
+
+window.wallpaper.visible {
+    opacity: 1;
 }
 
 /* Round avatar image */