feat(save): native OS dialog for Save As, keep in-app format picker

Replace the custom in-app file-browser save dialog with the native OS
save dialog for choosing the destination path — folder browsing and
overwrite confirmation now come from the OS. A compact in-app modal
still selects the DWG/DXF format and version (a native dialog cannot
carry the version year) and appears only for Save As. Plain Save on an
unsaved drawing defaults to DWG 2018 with no picker. The file-name field
is shown only on web, where there is no native dialog.

Remove the hand-rolled directory listing, folder navigation, and
overwrite modal.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -558,29 +558,16 @@ impl OpenCADStudio {
                 Task::none()
             }
 
-            Message::SaveDialogNavigate(path) => {
-                self.save_dialog_folder = path.clone();
-                self.save_dialog_entries = crate::io::read_dir_entries(&path);
-                Task::none()
-            }
-
-            Message::SaveDialogEntryClicked(path, is_dir) => {
-                if is_dir {
-                    self.save_dialog_folder = path.clone();
-                    self.save_dialog_entries = crate::io::read_dir_entries(&path);
-                } else {
-                    // Fill filename from clicked file.
-                    if let Some(name) = path.file_name() {
-                        self.save_dialog_filename = name.to_string_lossy().into_owned();
-                    }
-                }
-                Task::none()
-            }
-
             Message::SaveDialogConfirm => self.on_save_dialog_confirm(),
 
             Message::SaveDialogCancel => self.close_save_dialog_window(),
 
+            #[cfg(not(target_arch = "wasm32"))]
+            Message::SaveDialogPathPicked(picked) => self.on_save_dialog_path_picked(picked),
+
+            #[cfg(target_arch = "wasm32")]
+            Message::SaveDialogPathPicked(_) => Task::none(),
+
             Message::ClearScene => {
                 let i = self.active_tab;
                 self.push_undo_snapshot(i, "CLEAR");
@@ -3139,11 +3126,6 @@ impl OpenCADStudio {
                 Task::none()
             }
 
-            Message::OverwriteConfirm => self.on_overwrite_confirm(),
-            Message::OverwriteCancel => {
-                self.active_modal = Some(crate::app::ModalKind::SaveDialog);
-                Task::none()
-            }
             Message::AutoSave => self.on_autosave(),
 
             Message::UnsavedPickedSavePath(Some(path)) => {