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/App.tsx

@@ -0,0 +1,32 @@
+import React from 'react';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import { AppRouter } from '@/routes';
+import { setUnauthorizedHandler } from '@/api/client';
+import { useAuthStore } from '@/store/authStore';
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      retry: 1,
+      staleTime: 5 * 60 * 1000,
+      refetchOnWindowFocus: false,
+    },
+  },
+});
+
+export default function App() {
+  const { logout } = useAuthStore();
+
+  React.useEffect(() => {
+    setUnauthorizedHandler(() => {
+      logout();
+      window.location.href = '/login';
+    });
+  }, [logout]);
+
+  return (
+    <QueryClientProvider client={queryClient}>
+      <AppRouter />
+    </QueryClientProvider>
+  );
+}