feat: add set as wallpaper functionality

Add ability to set current image as desktop wallpaper with keyboard
shortcut 'W' and icon button in Properties panel.

Supports COSMIC, GNOME, KDE, XFCE, and tiling window managers via
automatic detection and fallback mechanism.

Implementation uses wallpaper crate with custom COSMIC config file
integration and gsettings/feh fallbacks.

src/app/message.rs

@@ -70,6 +70,10 @@ pub enum AppMessage {
     #[allow(dead_code)]
     RefreshMetadata,
 
+    // === Wallpaper ===
+    /// Set current image as wallpaper.
+    SetAsWallpaper,
+
     // === Errors ===
     /// Display an error message.
     #[allow(dead_code)]