Add rate limiting dashboard to admin panel

- Enhance GET /admin/rate-limits to return per-IP breakdown, 24h throttled
  count, and hourly time-series of rejected requests
- Add AdminRateLimits React page with auto-refresh (configurable interval),
  summary cards, throttled-over-time bar chart, endpoint table, and per-IP
  breakdown table
- Add TypeScript types (RateLimitStatsResponse, etc.) and adminApi.getRateLimits()
- Wire up /admin/rate-limits route and nav link (admin-only)
- Expand unit tests: auth, empty state, per-IP, throttled_24h, time-series,
  response shape contract (10 tests total)

Closes leeworks-agents/SPARC#1686

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

frontend/src/components/Layout.tsx

@@ -1,7 +1,7 @@
 import { Outlet, NavLink, useNavigate } from 'react-router-dom';
 import { useAuth } from '../context/AuthContext';
 import { useTheme } from '../context/ThemeContext';
-import { Search, Layers, BarChart3, Info, Users, LogOut, GitCompareArrows, Sun, Moon } from 'lucide-react';
+import { Search, Layers, BarChart3, Info, Users, LogOut, GitCompareArrows, Sun, Moon, History, ShieldAlert } from 'lucide-react';
 
 export function Layout() {
   const { user, isAdmin, logout } = useAuth();
@@ -18,11 +18,13 @@ export function Layout() {
     { to: '/batch', icon: Layers, label: 'Batch' },
     { to: '/analytics', icon: BarChart3, label: 'Analytics' },
     { to: '/compare', icon: GitCompareArrows, label: 'Compare' },
+    { to: '/history-diff', icon: History, label: 'Diff' },
     { to: '/about', icon: Info, label: 'About' },
   ];
 
   if (isAdmin) {
     navItems.push({ to: '/admin/users', icon: Users, label: 'Users' });
+    navItems.push({ to: '/admin/rate-limits', icon: ShieldAlert, label: 'Rate Limits' });
   }
 
   return (