fix(save): preserve 3D view and thumbnails

Persist camera, render state, and solid fallback geometry across saves. Generate previews from a clean live viewport and refresh recent-file thumbnails immediately.

src/app/update/viewport.rs

@@ -809,6 +809,7 @@ impl OpenCADStudio {
         // render mode; the model-layout tab style is untouched.
         if self.tabs[i].scene.set_active_viewport_render_mode(mode) {
             self.tabs[i].scene.bump_geometry_no_blocks();
+            self.tabs[i].dirty = true;
             self.command_line
                 .push_output(crate::tf!("Viewport visual style: {label}").as_ref());
             return Task::none();
@@ -827,6 +828,7 @@ impl OpenCADStudio {
         // Re-upload face3d fills on the next frame — the render
         // pipeline keys its upload cache off `geometry_epoch`.
         self.tabs[i].scene.bump_geometry_no_blocks();
+        self.tabs[i].dirty = true;
         self.command_line
             .push_output(crate::tf!("Visual style: {label}").as_ref());
         Task::none()