fix(config): use XDG picture directory as default

Replace hardcoded \"~/Pictures\" with dirs::picture_dir() which resolves
to the actual absolute path (e.g. /home/user/Pictures).

The tilde (~) in paths is not automatically expanded by Rust's PathBuf,
causing path.exists() to return false for \"~/Pictures\".

Using dirs::picture_dir() provides:
- Proper absolute path resolution
- XDG Base Directory compliance
- Fallback to home directory if Pictures doesn't exist

Note: Users with existing config files need to delete
~/.config/cosmic/org.codeberg.wfx.Noctua/v1/default_image_dir
to apply the new default."

src/config.rs

@@ -21,8 +21,7 @@ pub struct AppConfig {
 impl Default for AppConfig {
     fn default() -> Self {
         Self {
-            // TODO: Use xdg dir for picture
-            default_image_dir: Some(PathBuf::from("~/Pictures")),
+            default_image_dir: dirs::picture_dir().or_else(dirs::home_dir),
             nav_bar_visible: false,
             context_drawer_visible: false,
         }