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/theme.rs

@@ -93,6 +93,10 @@ pub struct Tokens {
     pub dialog_bg: Color32,
     pub dialog_border: Color32,
     pub chip: Color32,
+    /// Band behind a panel's heading and, lighter, behind its section headings (both set off
+    /// from `dialog_bg`).
+    pub heading_bg: Color32,
+    pub subheading_bg: Color32,
     /// Inside of bodies cut by a section.
     pub section_cap: Color32,
     /// Closed sketch profiles (translucent fill).
@@ -183,6 +187,8 @@ impl Tokens {
             dialog_bg: Color32::from_rgb(250, 251, 252),
             dialog_border: Color32::from_rgb(208, 212, 220),
             chip: Color32::from_rgb(38, 120, 218),
+            heading_bg: Color32::from_rgb(218, 223, 231),
+            subheading_bg: Color32::from_rgb(234, 237, 242),
             profile_fill: Color32::from_rgba_unmultiplied_const(255, 196, 120, 70),
             glyph_bg: Color32::WHITE,
             glyph_edge: Color32::from_rgb(90, 150, 90),
@@ -238,6 +244,8 @@ impl Tokens {
         t.dialog_bg = Color32::from_rgb(59, 68, 83);
         t.dialog_border = Color32::from_rgb(72, 83, 99);
         t.chip = Color32::from_rgb(59, 101, 168);
+        t.heading_bg = Color32::from_rgb(38, 44, 55);
+        t.subheading_bg = Color32::from_rgb(48, 55, 68);
         t.glyph_bg = Color32::from_rgb(44, 50, 60);
         t.glyph_edge = Color32::from_rgb(96, 170, 110);
         t.glyph_text = Color32::from_rgb(150, 220, 160);
@@ -300,6 +308,16 @@ pub fn apply(ctx: &egui::Context) {
 
 #[cfg(test)]
 mod tests {
+    /// Title and section bands stand out from a dialog, the title more than its sections.
+    #[test]
+    fn heading_bands_are_darker_than_the_dialog_in_order() {
+        let luma = |c: egui::Color32| u32::from(c.r()) * 3 + u32::from(c.g()) * 6 + u32::from(c.b());
+        for t in [super::Tokens::light(), super::Tokens::dark()] {
+            assert!(luma(t.heading_bg) < luma(t.subheading_bg), "{t:?}");
+            assert!(luma(t.subheading_bg) < luma(t.dialog_bg), "{t:?}");
+        }
+    }
+
     #[test]
     fn active_movable_window_header_uses_theme_panel_header() {
         for dark in [false, true] {