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.

docs/features.md

@@ -104,6 +104,10 @@ This document describes the implemented and planned features of Noctua, a modern
 - **Properties panel**:
   - Image metadata display
   - File information
+  - Action buttons:
+    - Set as Wallpaper (works with COSMIC, GNOME, KDE, XFCE, and tiling WMs)
+    - Open With… (planned)
+    - Show in Folder (planned)
   - Toggle with `i` key or toolbar button
 - **Navigation panel** (Left sidebar):
   - Toggle with `n` key or toolbar button
@@ -116,6 +120,23 @@ Full keyboard-driven workflow:
 - Pan: `Ctrl + ←` `Ctrl + →` `Ctrl + ↑` `Ctrl + ↓`
 - Transform: `r` `Shift+r` `h` `v`
 - Panels: `i` `n`
+- Actions: `w` (Set as Wallpaper)
+
+### Desktop Integration
+
+#### Wallpaper Support (Implemented)
+- **Set as Wallpaper**: One-click wallpaper setting with cross-desktop compatibility
+- **Supported desktop environments**:
+  - COSMIC Desktop (direct config file integration)
+  - GNOME (via gsettings)
+  - KDE Plasma (via wallpaper crate)
+  - XFCE (via wallpaper crate)
+  - Tiling window managers (via feh)
+- **Multiple access methods**:
+  - Keyboard shortcut: `w`
+  - Icon button in Properties panel
+  - Tooltip support for discoverability
+- **Automatic fallback**: Tries multiple methods until one succeeds
 
 ### Configuration