fix(frontend): code-review polish — replaceState, arrow-key nav, TabId, comment

Fix 4: useHashTab navigate() uses history.replaceState instead of
window.location.hash assignment — no spurious history entries, no
anchor-jump scroll side-effect.
Fix 5: Tabs ArrowLeft/ArrowRight keyboard nav (WAI-ARIA tabs pattern).
Fix 6: TabId union type in EngagementDetailPage, cast from string for
type-safe switch on activeTab without breaking Tabs.onChange signature.
Fix 7: intentional comment on UsersAdminPage reset-password aerated row.
Tests: 236/236 (+2 arrow-key assertions in Tabs.test.tsx).

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

frontend/src/pages/UsersAdminPage.tsx

@@ -248,6 +248,7 @@ export function UsersAdminPage(): JSX.Element {
                       </tr>
                       {resetOpen === u.id ? (
                         <tr className="border-b border-hairline last:border-0 bg-cloud">
+                          {/* Aerated row — inline form needs room; compact-density tradeoff intentional */}
                           <td colSpan={4} className="px-xl py-md">
                             <form
                               onSubmit={(e) => onResetPassword(u, e)}