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/document/mod.rs

@@ -110,3 +110,35 @@ impl DocumentContent {
         }
     }
 }
+
+/// Set an image file as desktop wallpaper.
+///
+/// This function attempts multiple methods in order:
+/// 1. COSMIC Desktop (direct config file modification)
+/// 2. wallpaper crate (KDE, XFCE, Windows, macOS)
+/// 3. gsettings (GNOME)
+/// 4. feh (tiling window managers)
+///
+/// The operation is performed asynchronously and logs success/failure.
+pub fn set_as_wallpaper(path: &Path) {
+    // Canonicalize to absolute path
+    let abs_path = match path.canonicalize() {
+        Ok(p) => p,
+        Err(e) => {
+            log::error!("Failed to canonicalize path {}: {}", path.display(), e);
+            return;
+        }
+    };
+
+    // Convert to string
+    let path_str = match abs_path.to_str() {
+        Some(s) => s.to_string(),
+        None => {
+            log::error!("Invalid UTF-8 in path: {}", abs_path.display());
+            return;
+        }
+    };
+
+    // Delegate to utils with concrete string type
+    utils::set_as_wallpaper(&path_str);
+}