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/App.tsx

@@ -11,7 +11,9 @@ import { Batch } from './pages/Batch';
 import { AnalyticsPage } from './pages/Analytics';
 import { About } from './pages/About';
 import { AdminUsers } from './pages/AdminUsers';
+import { AdminRateLimits } from './pages/AdminRateLimits';
 import { Compare } from './pages/Compare';
+import { HistoryDiff } from './pages/HistoryDiff';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -45,6 +47,7 @@ function App() {
               <Route path="/batch" element={<Batch />} />
               <Route path="/analytics" element={<AnalyticsPage />} />
               <Route path="/compare" element={<Compare />} />
+              <Route path="/history-diff" element={<HistoryDiff />} />
               <Route path="/about" element={<About />} />
 
               {/* Admin routes */}
@@ -56,6 +59,14 @@ function App() {
                   </ProtectedRoute>
                 }
               />
+              <Route
+                path="/admin/rate-limits"
+                element={
+                  <ProtectedRoute requireAdmin>
+                    <AdminRateLimits />
+                  </ProtectedRoute>
+                }
+              />
             </Route>
 
             {/* Default redirect */}