M1: save a copy as .fig

Save as now offers .figcraft and .fig as separate file types. A .fig
destination, chosen in the dialog or passed to file.saveAs and file.save,
writes the existing experimental .fig export of the document that is shown
and reports how much was left out. The file's own path, unsaved changes,
versions, branches and undo history are not touched, and a shown earlier
version stays shown while the copy is written or a Save as choice is
pending.

Plain Save no longer writes over a file whose saved path ends in .fig;
earlier builds could write native data under that name. It asks for a
destination instead.

apps/figcraft/src/services.rs

@@ -223,12 +223,25 @@ fn picked(pick: impl Future<Output = Option<rfd::FileHandle>> + 'static) -> Dial
     Box::pin(async move { pick.await.map(|f| f.path().to_path_buf()) })
 }
 
+/// Native Save as filters; other callers keep their existing combined filter.
+fn save_filters<'a>(exts: &'a [&'a str]) -> Vec<(&'static str, &'a [&'a str])> {
+    if exts == ["figcraft", "fig"] {
+        vec![("FigCraft (.figcraft)", &exts[..1]), (figcraft_ui_egui::i18n::tr(".fig (experimental)"), &exts[1..])]
+    } else {
+        vec![("File", exts)]
+    }
+}
+
 impl Services for Desktop {
     fn pick_open(&mut self, title: &str, exts: &[&str]) -> DialogFuture {
         picked(dialog(title).add_filter("Supported files", exts).pick_file())
     }
     fn pick_save(&mut self, title: &str, name: &str, exts: &[&str]) -> DialogFuture {
-        picked(dialog(title).set_file_name(name).add_filter("File", exts).save_file())
+        let mut dialog = dialog(title).set_file_name(name);
+        for (label, extensions) in save_filters(exts) {
+            dialog = dialog.add_filter(label, extensions);
+        }
+        picked(dialog.save_file())
     }
     fn pick_folder(&mut self, title: &str) -> DialogFuture {
         picked(dialog(title).pick_folder())
@@ -297,3 +310,24 @@ pub fn window_icon() -> Option<egui::IconData> {
     let (w, h) = img.dimensions();
     Some(egui::IconData { rgba: img.into_raw(), width: w, height: h })
 }
+
+#[cfg(test)]
+mod save_filter_tests {
+    use super::save_filters;
+
+    #[test]
+    fn native_and_fig_save_filters_are_separate_with_native_first() {
+        let extensions = ["figcraft", "fig"];
+        assert_eq!(
+            save_filters(&extensions),
+            vec![("FigCraft (.figcraft)", &extensions[..1]), (figcraft_ui_egui::i18n::tr(".fig (experimental)"), &extensions[1..])]
+        );
+    }
+
+    #[test]
+    fn other_save_filters_keep_one_combined_group() {
+        for extensions in [&["figcraft"][..], &["fig"][..], &["png"][..], &["png", "jpg", "webp"][..], &[][..]] {
+            assert_eq!(save_filters(extensions), vec![("File", extensions)]);
+        }
+    }
+}