feat(ui): heading bands for dialogs, sketch status label, tighter palette footer (#418)

- frame::heading draws a panel's title row (heading_bg) or a section heading
  (subheading_bg) on a band darker than the dialog, in both themes. Every
  docked command dialog, the Sketch Palette and Redefine Sketch Plane use it.
- Sketch Palette: "Options" and a new "Sketch Status" heading are section
  bands; the status line sits under its own heading.
- Sketch Palette: Finish Sketch no longer centres in the leftover height, so
  the palette ends one line below it, one line after the status.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: bherbruck <brennen.herbruck@gmail.com>

crates/ui-egui/src/dialogs.rs

@@ -1350,8 +1350,9 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
         ui.set_max_width(if wide { DIALOG_WIDE_MAX_W } else { DIALOG_MAX_W } - margin);
         // Rows that end at the right edge line up with last frame's content width.
         let total = crate::frame::content_width(ui, wkey, min_w);
-        // Header: "−" folds the dialog, the command name, "»" at the right.
-        ui.horizontal(|ui| {
+        // Header: "−" folds the dialog, the command name, "»" at the right, on the title band.
+        crate::frame::heading(ui, crate::frame::Level::Title, total, |ui| {
+            let total = total - 2.0 * crate::frame::HEADING_MARGIN;
             let fold = if d.collapsed { "+" } else { "−" };
             if ui
                 .add(egui::Button::new(RichText::new(fold).size(14.0)).frame(false))