T07a: frontend core SPA — shell + auth + routing + i18n + UI library + a11y

- React 18 + Vite + TypeScript + Tailwind CSS setup
- AppShell with Sidebar (plugin menu) + TopBar (tenant switcher, search, notifications, user menu)
- Auth pages: Login, PasswordResetRequest, PasswordResetConfirm
- Protected routes with auth guard
- API client (axios with interceptors: session cookie, 401 redirect, 422 validation)
- TanStack Query hooks for auth, users, companies, contacts, notifications
- Zustand stores: authStore, uiStore
- i18n setup (de/en locales) with react-i18next
- UI component library: Button, Input, Select, Modal, Toast, Table, Card, Badge, Avatar, Pagination, EmptyState, Skeleton, ConfirmDialog
- Accessibility: ARIA labels, 44px touch targets, keyboard nav, reduced-motion, sr-only
- Design tokens from prototype as CSS custom properties
- 111 tests passing across 20 test files
- tsc --noEmit: 0 errors
- npm run build: success (471KB JS, 24KB CSS)

frontend/src/routes/index.tsx

@@ -0,0 +1,40 @@
+import React from 'react';
+import { createBrowserRouter, RouterProvider } from 'react-router-dom';
+import { AppShell } from '@/components/layout/AppShell';
+import { ProtectedRoute } from './ProtectedRoute';
+import { LoginPage } from '@/pages/Login';
+import { PasswordResetRequestPage } from '@/pages/PasswordResetRequest';
+import { PasswordResetConfirmPage } from '@/pages/PasswordResetConfirm';
+import { DashboardPage } from '@/pages/Dashboard';
+import { SettingsPage } from '@/pages/Settings';
+
+const router = createBrowserRouter([
+  {
+    path: '/login',
+    element: <LoginPage />,
+  },
+  {
+    path: '/password-reset',
+    element: <PasswordResetRequestPage />,
+  },
+  {
+    path: '/password-reset/confirm',
+    element: <PasswordResetConfirmPage />,
+  },
+  {
+    element: (
+      <ProtectedRoute>
+        <AppShell />
+      </ProtectedRoute>
+    ),
+    children: [
+      { path: '/dashboard', element: <DashboardPage /> },
+      { path: '/settings', element: <SettingsPage /> },
+      { path: '/', element: <DashboardPage /> },
+    ],
+  },
+]);
+
+export function AppRouter() {
+  return <RouterProvider router={router} />;
+}