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."

Cargo.toml

@@ -38,6 +38,7 @@ i18n-embed-fl = "0.10"
 # Misc utilities
 open = "5.3.2"
 rust-embed = "8.8.0"
+dirs = "5.0"
 image = "0.25.9"
 clap = { version = "4.5.54", features = ["derive"] }
 env_logger = "0.11.8"