feat(T01): auth + user management + RBAC + base frontend + i18n

- Backend: FastAPI, JWT auth (HS256), bcrypt, RBAC middleware
- User CRUD (admin-only), soft-delete, pagination
- Pydantic BaseSettings config, async SQLAlchemy 2.0
- 50 backend tests, 88% coverage
- Frontend: Next.js 14 App Router, Tailwind, design tokens
- Login page, auth context, API client with auto-refresh
- i18n: next-intl, DE/EN (31 keys each)
- 6 base UI components (Button, Input, Card, Table, Modal, Toast)
- 12 frontend tests, npm build success

frontend/lib/i18n.tsx

@@ -0,0 +1,60 @@
+'use client';
+
+import { createContext, useContext, useState, useCallback, type ReactNode } from 'react';
+import deMessages from '@/messages/de.json';
+import enMessages from '@/messages/en.json';
+
+export type Locale = 'de' | 'en';
+
+interface Messages {
+  [key: string]: string;
+}
+
+const messageMap: Record<Locale, Messages> = {
+  de: deMessages as Messages,
+  en: enMessages as Messages,
+};
+
+interface I18nContextValue {
+  locale: Locale;
+  setLocale: (locale: Locale) => void;
+  t: (key: string, params?: Record<string, string>) => string;
+}
+
+const I18nContext = createContext<I18nContextValue | null>(null);
+
+export function I18nProvider({ children, initialLocale = 'de' }: { children: ReactNode; initialLocale?: Locale }) {
+  const [locale, setLocaleState] = useState<Locale>(initialLocale);
+
+  const setLocale = useCallback((newLocale: Locale) => {
+    setLocaleState(newLocale);
+    if (typeof document !== 'undefined') {
+      document.documentElement.lang = newLocale;
+    }
+  }, []);
+
+  const t = useCallback((key: string, params?: Record<string, string>) => {
+    const messages = messageMap[locale];
+    let message = messages[key] || key;
+    if (params) {
+      for (const [param, value] of Object.entries(params)) {
+        message = message.replace(new RegExp(`\{${param}\}`, 'g'), value);
+      }
+    }
+    return message;
+  }, [locale]);
+
+  return (
+    <I18nContext.Provider value={{ locale, setLocale, t }}>
+      {children}
+    </I18nContext.Provider>
+  );
+}
+
+export function useI18n(): I18nContextValue {
+  const ctx = useContext(I18nContext);
+  if (!ctx) {
+    throw new Error('useI18n must be used within I18nProvider');
+  }
+  return ctx;
+}