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/__tests__/ui/i18n.test.ts

@@ -0,0 +1,35 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import i18n from '@/i18n';
+
+describe('i18n', () => {
+  beforeAll(async () => {
+    await i18n.changeLanguage('de');
+  });
+
+  it('loads German locale by default', () => {
+    expect(i18n.language).toBe('de');
+  });
+
+  it('translates key in German', () => {
+    expect(i18n.t('nav.dashboard')).toBe('Dashboard');
+    expect(i18n.t('nav.companies')).toBe('Firmen');
+    expect(i18n.t('auth.login')).toBe('Anmelden');
+  });
+
+  it('can switch to English', async () => {
+    await i18n.changeLanguage('en');
+    expect(i18n.language).toBe('en');
+    expect(i18n.t('auth.login')).toBe('Sign In');
+    expect(i18n.t('nav.companies')).toBe('Companies');
+  });
+
+  it('falls back to German for missing English keys', async () => {
+    await i18n.changeLanguage('en');
+    expect(i18n.t('app.name')).toBe('leocrm');
+  });
+
+  it('switches back to German', async () => {
+    await i18n.changeLanguage('de');
+    expect(i18n.t('auth.login')).toBe('Anmelden');
+  });
+});