Add dd0c/route Dashboard UI: React + Vite + Tailwind SPA

- Layout with sidebar navigation (Dashboard, Rules, Keys, Settings)
- Dashboard page: stat cards, cost savings area chart (Recharts), model usage table
- Rules page: routing rules CRUD with modal editor, strategy/complexity/model matching
- Keys page: API key generation, copy-once reveal, revocation, quick-start code snippet
- Settings page: org info, provider config, danger zone
- API client (SWR + fetch wrapper) with full TypeScript types
- dd0c dark theme: indigo primary, cyan accent, dark surfaces
- Vite proxy config for local dev against API on :3000

products/01-llm-cost-router/ui/src/App.tsx

@@ -0,0 +1,20 @@
+import { Routes, Route, Navigate } from 'react-router-dom';
+import Layout from './components/Layout';
+import Dashboard from './pages/Dashboard';
+import Rules from './pages/Rules';
+import Keys from './pages/Keys';
+import Settings from './pages/Settings';
+
+export default function App() {
+  return (
+    <Routes>
+      <Route element={<Layout />}>
+        <Route path="/" element={<Navigate to="/dashboard" replace />} />
+        <Route path="/dashboard" element={<Dashboard />} />
+        <Route path="/rules" element={<Rules />} />
+        <Route path="/keys" element={<Keys />} />
+        <Route path="/settings" element={<Settings />} />
+      </Route>
+    </Routes>
+  );
+}