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/auth.ts

@@ -0,0 +1,80 @@
+'use client';
+
+import { useRouter } from 'next/navigation';
+import { useEffect, useState, useCallback } from 'react';
+import {
+  login as apiLogin,
+  getCurrentUser,
+  setTokens,
+  clearTokens,
+  isAuthenticated,
+  type UserResponse,
+  type ApiError,
+} from './api';
+
+export interface AuthState {
+  user: UserResponse | null;
+  loading: boolean;
+  error: string | null;
+}
+
+export function useAuth() {
+  const [state, setState] = useState<AuthState>({
+    user: null,
+    loading: true,
+    error: null,
+  });
+
+  const fetchUser = useCallback(async () => {
+    if (!isAuthenticated()) {
+      setState({ user: null, loading: false, error: null });
+      return;
+    }
+    try {
+      const user = await getCurrentUser();
+      setState({ user, loading: false, error: null });
+    } catch {
+      clearTokens();
+      setState({ user: null, loading: false, error: 'Session expired' });
+    }
+  }, []);
+
+  useEffect(() => {
+    fetchUser();
+  }, [fetchUser]);
+
+  const login = useCallback(async (email: string, password: string) => {
+    setState({ user: null, loading: true, error: null });
+    try {
+      const tokens = await apiLogin(email, password);
+      setTokens(tokens);
+      const user = await getCurrentUser();
+      setState({ user, loading: false, error: null });
+      return user;
+    } catch (err) {
+      const message = (err as ApiError)?.error?.message || 'Login failed';
+      setState({ user: null, loading: false, error: message });
+      throw err;
+    }
+  }, []);
+
+  const logout = useCallback(() => {
+    clearTokens();
+    setState({ user: null, loading: false, error: null });
+  }, []);
+
+  return { ...state, login, logout, refresh: fetchUser };
+}
+
+export function useRequireAuth() {
+  const auth = useAuth();
+  const router = useRouter();
+
+  useEffect(() => {
+    if (!auth.loading && !auth.user) {
+      router.push('/login');
+    }
+  }, [auth.loading, auth.user, router]);
+
+  return auth;
+}