fix: drop RefCell borrow during SVG saving and configure FileDialog to default to SVG

src/main.rs

@@ -193,14 +193,25 @@ fn build_editor(stack: &Stack, window: &ApplicationWindow, icon_path: PathBuf, b
                     let window_clone = window.clone();
                     let comp = Rc::clone(&comp);
                     move || {
+                        let filter = gtk::FileFilter::new();
+                        filter.set_name(Some("SVG Images"));
+                        filter.add_mime_type("image/svg+xml");
+                        filter.add_pattern("*.svg");
+                        
+                        let filters = gtk::gio::ListStore::new::<gtk::FileFilter>();
+                        filters.append(&filter);
+
                         let dialog = gtk::FileDialog::builder()
                             .title("Save Composite Icon As...")
+                            .filters(&filters)
+                            .default_filter(&filter)
                             .build();
                         let c = Rc::clone(&comp);
                         dialog.save(Some(&window_clone), gtk::gio::Cancellable::NONE, move |res| {
                             if let Ok(file) = res {
                                 if let Some(path) = file.path() {
-                                    if let Err(e) = std::fs::write(&path, c.borrow().render()) {
+                                    let rendered = c.borrow().render();
+                                    if let Err(e) = std::fs::write(&path, rendered) {
                                         eprintln!("Failed to save composite to {}: {e}", path.display());
                                     } else {
                                         c.borrow_mut().is_modified = false;