fix(frontend): design-review polish — table-compact height, header type-scale, tabs a11y

Fix 1 (A): move row height constraint from min-height on tbody tr
(CSS no-op on table-row) to height: 32px on td (works on table-cell).
Fix 2 (A): table header text-[11px] -> text-[12px] to align with
documented caption scale.
Fix 3: add aria-controls + id to Tabs buttons; wrap active tab content
in role="tabpanel" + aria-labelledby in EngagementDetailPage.
Test: add aria-controls/id assertion to Tabs.test.tsx (234 total).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/Tabs.tsx

@@ -21,8 +21,10 @@ export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
         return (
           <button
             key={item.id}
+            id={`tab-${item.id}`}
             role="tab"
             aria-selected={isActive}
+            aria-controls={`tabpanel-${item.id}`}
             type="button"
             onClick={() => onChange(item.id)}
             className={`tab-underline${isActive ? ' tab-underline-active' : ''} pb-xs`}