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/hooks/useHashTab.ts

@@ -17,7 +17,8 @@ export function useHashTab(defaultId: string): [string, (id: string) => void] {
   }, [defaultId]);
 
   const navigate = useCallback((id: string) => {
-    window.location.hash = id;
+    // replaceState: no history entry (Back button unaffected), no anchor-jump scroll
+    history.replaceState(null, '', '#' + id);
     setActiveId(id);
   }, []);