feat(sw): refine viewer model layout design and theme tokens

- Model / Layout tabs follow Figma 7652:40344: gradient faces, gold / green
  gradient borders, blurred outline, inner shadow and top gloss, with every
  color in its own --hm-sw-viewer-tab(s)-* token (consumer-overridable)
- Box height matches the adjacent field group (42px, from the control height
  token); padding 1px 1px 1.5px, button side padding 5px, 1px right margin
  for a tabs-only first group; 378px max width, equal buttons and the
  measured More steps are unchanged; existing text-shadow kept
- Storybook: SW/Viewer > Model / Layout story (states and token override);
  Logo story listed before Responsive More
- AI docs (VIEWER.md, DESIGN_RULES.md) and viewer / text-weight test
  expectations updated

packages/sw/tests/viewer.browser.mjs

@@ -82,8 +82,8 @@ if (reference && reference !== '-') {
   await page.close()
 }
 // 2026-10-23: the Model / Layout buttons share one width (their field face widens up to 378px with room), so they and
-// their field are left out of the size comparison with the reference; height and look are unchanged.
-check('geometry tabs: equal widths, 40px high', comparison.package.tab.length === 2 && comparison.package.tab.every(item => item.h === 40 && Math.abs(item.w - comparison.package.tab[0].w) <= 1), comparison.package.tab.map(item => `${item.w}x${item.h}`).join(' '))
+// their field are left out of the size comparison with the reference. Height: the box matches the field group (42px), so the buttons are 37.5px (margin 1px top and sides, 1.5px bottom; geometry rounds to 38).
+check('geometry tabs: equal widths, 37.5px high (box = field group 42px)', comparison.package.tab.length === 2 && comparison.package.tab.every(item => item.h === 38 && Math.abs(item.w - comparison.package.tab[0].w) <= 1), comparison.package.tab.map(item => `${item.w}x${item.h}`).join(' '))
 if (comparison.reference) {
   // Same item = same size (positions shift with the file-name text width, which differs by sample data).
   const noTabs = items => items.filter(item => !item.text.startsWith('Model'))