M3.57: save swatch libraries as .vcswatches, .gpl or CSS and load them back

Save Swatch Library… (Swatches panel menu, libraries menu, Window menu)
opens a dialog: name, format and destination. .vcswatches is lossless JSON
(colour models, global, spot, gradients, groups), .gpl an RGB palette with
groups as comment headers, CSS custom properties with escaped names.
Libraries saved to the user library folder show under User Defined.
Other Library… (and opening a .vcswatches/.gpl file) loads a library or
another document's swatches into the library panel.

Commands: swatch.library.save {path?, format?, names?, name?, user?}
(no path returns {data}), swatch.library.load {path?|data?|dataBase64?,
name?}; UI: window.swatchLibrary.other, ui.saveSwatchLibrary (dialog
saveSwatchLibrary), window.userSwatchLibrary1..10.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/vectorcraft/src/main.rs

@@ -161,6 +161,9 @@ fn main() -> eframe::Result {
         Box::new(move |cc| {
             let mut app = VectorcraftApp::new(Session::new(), services());
             load_prefs(&mut app);
+            // User Defined swatch libraries live next to the preferences.
+            let swatches = prefs_path().and_then(|p| Some(p.parent()?.join("Swatches").to_string_lossy().to_string()));
+            app.session.swatch_libraries.set_user_dir(swatches);
             app.integrated_titlebar = cfg!(target_os = "macos");
             if let Some(port) = control_port {
                 let rx = control_server::start(port, cc.egui_ctx.clone());