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.

CHANGELOG.md

@@ -5,6 +5,10 @@ Format based on [Keep a Changelog](https://keepachangelog.com/).
 
 ## [0.4.1] - 2026-03-28
 
+### Added
+
+- Fade-in animation (250ms ease-in) for panel and wallpaper windows via CSS opacity transition
+
 ### Fixed
 
 - Fix pixel format mismatch in blur path — `texture.download()` yields BGRA but was passed to `RgbaImage` without channel swap, now explicitly converts B↔R