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/i18n/index.ts

@@ -0,0 +1,27 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import LanguageDetector from 'i18next-browser-languagedetector';
+import de from './locales/de.json';
+import en from './locales/en.json';
+
+i18n
+  .use(LanguageDetector)
+  .use(initReactI18next)
+  .init({
+    resources: {
+      de: { translation: de },
+      en: { translation: en },
+    },
+    fallbackLng: 'de',
+    supportedLngs: ['de', 'en'],
+    interpolation: {
+      escapeValue: false,
+    },
+    detection: {
+      order: ['localStorage', 'navigator'],
+      lookupLocalStorage: 'leocrm_lang',
+      caches: ['localStorage'],
+    },
+  });
+
+export default i18n;