feat(ribbon): tighten + squeeze collapsed panels, content-driven height

Adaptive tool-panel row now keeps degrading past the collapsed state instead
of overflowing:

- Panels still shrink from the right one at a time (full -> compact ->
  collapsed). Once every panel is a collapsed button and the row still
  overflows, each button drops its large icon to a small one (tight) together,
  then the buttons are pulled together up to 8px per gap (reclaiming edge
  padding) before anything is clipped.
- Tool-area height now tracks the tallest shown panel, so the ribbon shrinks
  as its panels collapse; the measured height is reported for dropdown
  anchoring.
- Tabs squeeze their inter-tab gaps shut before wrapping to a new row
  (WrapFlow min_spacing_x).

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

src/ui/ribbon/widgets.rs

@@ -37,6 +37,10 @@ pub(super) const ARROW_W: f32 = ROW_H * 0.4;
 pub(super) const LARGE_ARR: f32 = ROW_H * 0.55;
 /// Total ribbon tool-area height = 3 × ROW_H + 6 px v-padding + 12 px group-label.
 pub(super) const TOOL_BAR_H: f32 = 3.0 * ROW_H + 18.0;
+/// Height of a collapsed panel button's large representative face (big icon +
+/// its label). A collapsed button is this face plus the title opener, so it is
+/// shorter than a full 3-row panel — the ribbon height follows it down.
+pub(super) const COLLAPSED_FACE_H: f32 = LARGE_ICON + 20.0;
 
 // ── Tab-bar constants ──────────────────────────────────────────────────────