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/test/setup.ts

@@ -0,0 +1,66 @@
+import '@testing-library/jest-dom';
+import { vi, beforeEach, afterEach, beforeAll } from 'vitest';
+import i18n from '@/i18n';
+
+// Mock matchMedia for jsdom
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: vi.fn().mockImplementation((query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+});
+
+// Mock IntersectionObserver
+class MockIntersectionObserver {
+  observe = vi.fn();
+  unobserve = vi.fn();
+  disconnect = vi.fn();
+  takeRecords = vi.fn();
+  root = null;
+  rootMargin = '';
+  thresholds = [];
+}
+
+Object.defineProperty(window, 'IntersectionObserver', {
+  writable: true,
+  configurable: true,
+  value: MockIntersectionObserver,
+});
+
+// Mock ResizeObserver
+class MockResizeObserver {
+  observe = vi.fn();
+  unobserve = vi.fn();
+  disconnect = vi.fn();
+}
+
+Object.defineProperty(window, 'ResizeObserver', {
+  writable: true,
+  configurable: true,
+  value: MockResizeObserver,
+});
+
+// Mock scrollTo
+window.scrollTo = vi.fn() as any;
+
+// Initialize i18n before all tests
+beforeAll(async () => {
+  await i18n.changeLanguage('de');
+});
+
+// Clean up after each test
+afterEach(() => {
+  vi.clearAllMocks();
+  localStorage.clear();
+});
+
+beforeEach(() => {
+  localStorage.clear();
+});