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/api.ts

@@ -0,0 +1,147 @@
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000/api/v1';
+
+export interface ApiError {
+  error: {
+    code: string;
+    message: string;
+    details?: unknown;
+  };
+}
+
+export interface PaginatedResponse<T> {
+  items: T[];
+  total: number;
+  page: number;
+  page_size: number;
+}
+
+export interface TokenResponse {
+  access_token: string;
+  refresh_token: string;
+  token_type: string;
+  expires_in: number;
+}
+
+export interface UserResponse {
+  id: string;
+  email: string;
+  full_name: string;
+  role: string;
+  language: string;
+  is_active: boolean;
+  created_at?: string;
+  updated_at?: string;
+}
+
+function getAuthToken(): string | null {
+  if (typeof window === 'undefined') return null;
+  return localStorage.getItem('access_token');
+}
+
+function getRefreshToken(): string | null {
+  if (typeof window === 'undefined') return null;
+  return localStorage.getItem('refresh_token');
+}
+
+export function setTokens(tokens: TokenResponse): void {
+  if (typeof window === 'undefined') return;
+  localStorage.setItem('access_token', tokens.access_token);
+  localStorage.setItem('refresh_token', tokens.refresh_token);
+}
+
+export function clearTokens(): void {
+  if (typeof window === 'undefined') return;
+  localStorage.removeItem('access_token');
+  localStorage.removeItem('refresh_token');
+}
+
+export function isAuthenticated(): boolean {
+  return getAuthToken() !== null;
+}
+
+async function apiFetch<T>(
+  path: string,
+  options: RequestInit = {}
+): Promise<T> {
+  const token = getAuthToken();
+  const headers: Record<string, string> = {
+    'Content-Type': 'application/json',
+    ...((options.headers as Record<string, string>) || {}),
+  };
+
+  if (token) {
+    headers['Authorization'] = `Bearer ${token}`;
+  }
+
+  const response = await fetch(`${API_BASE_URL}${path}`, {
+    ...options,
+    headers,
+  });
+
+  if (response.status === 401 && token) {
+    const refreshToken = getRefreshToken();
+    if (refreshToken) {
+      const refreshResult = await apiFetch<TokenResponse>('/auth/refresh', {
+        method: 'POST',
+        body: JSON.stringify({ refresh_token: refreshToken }),
+      });
+      setTokens(refreshResult);
+      headers['Authorization'] = `Bearer ${refreshResult.access_token}`;
+      const retryResponse = await fetch(`${API_BASE_URL}${path}`, {
+        ...options,
+        headers,
+      });
+      if (!retryResponse.ok) {
+        const err = await retryResponse.json().catch(() => ({ error: { code: 'UNKNOWN', message: 'Request failed' } }));
+        throw err as ApiError;
+      }
+      return retryResponse.json();
+    }
+    clearTokens();
+  }
+
+  if (!response.ok) {
+    const err = await response.json().catch(() => ({ error: { code: 'UNKNOWN', message: 'Request failed' } }));
+    throw err as ApiError;
+  }
+
+  return response.json();
+}
+
+export async function login(email: string, password: string): Promise<TokenResponse> {
+  return apiFetch<TokenResponse>('/auth/login', {
+    method: 'POST',
+    body: JSON.stringify({ email, password }),
+  });
+}
+
+export async function getCurrentUser(): Promise<UserResponse> {
+  return apiFetch<UserResponse>('/auth/me');
+}
+
+export async function listUsers(page = 1, pageSize = 20): Promise<PaginatedResponse<UserResponse>> {
+  return apiFetch<PaginatedResponse<UserResponse>>(`/users/?page=${page}&page_size=${pageSize}`);
+}
+
+export async function createUser(data: {
+  email: string;
+  password: string;
+  full_name: string;
+  role: string;
+  language: string;
+}): Promise<UserResponse> {
+  return apiFetch<UserResponse>('/users/', {
+    method: 'POST',
+    body: JSON.stringify(data),
+  });
+}
+
+export async function deleteUser(userId: string): Promise<UserResponse> {
+  return apiFetch<UserResponse>(`/users/${userId}`, { method: 'DELETE' });
+}
+
+export async function healthCheck(): Promise<{ status: string }> {
+  return apiFetch<{ status: string }>('/health');
+}
+
+export { API_BASE_URL };