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/view/overlay.rs

@@ -702,6 +702,8 @@ pub(super) fn mtext_editor_overlay<'a>(
     // transparent fill lets the dimmed viewport show through beneath.
     crate::ui::modal::modal(
         iced::widget::Space::new().width(Fill).height(Fill),
+        "Text Editor",
+        660.0 + modal_resize.x,
         content,
         Message::MTextCancel,
         modal_offset,