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."
