feat(mtext): segmented Edit/Preview toggle, default to preview

The editor opens in preview by default and switches body views with a
segmented Edit | Preview control placed between the toolbar and the body
(active tab highlighted). All toolbar properties feed the rendered preview.

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

src/app/update.rs

@@ -3539,9 +3539,9 @@ impl OpenCADStudio {
                 self.rebuild_mtext_preview();
                 Task::none()
             }
-            Message::MTextTogglePreview => {
+            Message::MTextShowPreview(on) => {
                 if let Some(ed) = self.mtext_editor.as_mut() {
-                    ed.show_preview = !ed.show_preview;
+                    ed.show_preview = on;
                 }
                 self.rebuild_mtext_preview();
                 Task::none()