fix(styles): re-seed missing Standard styles on open; titled modals (#366)

Deleting/renaming Standard was already blocked in the five style
managers, but a file saved without the built-ins (foreign or damaged)
opened with broken style dropdowns and nothing to reference — re-seed
missing Standard text/dim/table/mleader/mline styles on every open.

Every modal dialog now shows its name in the title bar: centred at
15px across the dialog width, with the move grip and close button
overlaid at the right edge. The bar takes an explicit width from the
caller — a Fill child inside the Shrink frame would blow the dialog
out to the full screen.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/update/file.rs

@@ -435,6 +435,11 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
 
                 self.tabs[i].current_path = Some(path.clone());
                 self.tabs[i].scene.document = doc;
+                // A file saved without the built-in Standard styles (foreign
+                // or damaged) gets them re-seeded so nothing dangles (#366).
+                crate::app::style_ops::ensure_standard_styles(
+                    &mut self.tabs[i].scene.document,
+                );
                 // Follow the file's saved current UCS from the moment it opens.
                 self.tabs[i].adopt_active_ucs_from_header();
                 // Adopt the drawing's own Ortho ($ORTHOMODE) and running OSNAP