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/components/Tabs.tsx

@@ -1,3 +1,5 @@
+import type { KeyboardEvent } from 'react';
+
 interface TabItem {
   id: string;
   label: string;
@@ -11,12 +13,22 @@ interface TabsProps {
 }
 
 export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
+  function handleKeyDown(e: KeyboardEvent<HTMLButtonElement>, index: number) {
+    if (e.key === 'ArrowRight') {
+      e.preventDefault();
+      onChange(items[(index + 1) % items.length].id);
+    } else if (e.key === 'ArrowLeft') {
+      e.preventDefault();
+      onChange(items[(index - 1 + items.length) % items.length].id);
+    }
+  }
+
   return (
     <div
       role="tablist"
       className="flex items-end border-b border-hairline gap-xs"
     >
-      {items.map((item) => {
+      {items.map((item, index) => {
         const isActive = item.id === activeId;
         return (
           <button
@@ -27,6 +39,7 @@ export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
             aria-controls={`tabpanel-${item.id}`}
             type="button"
             onClick={() => onChange(item.id)}
+            onKeyDown={(e) => handleKeyDown(e, index)}
             className={`tab-underline${isActive ? ' tab-underline-active' : ''} pb-xs`}
           >
             {item.label}