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/components/ui/Modal.tsx

@@ -0,0 +1,40 @@
+import { ReactNode, useEffect } from 'react';
+
+interface ModalProps {
+  open: boolean;
+  onClose: () => void;
+  title?: string;
+  children: ReactNode;
+}
+
+export function Modal({ open, onClose, title, children }: ModalProps) {
+  useEffect(() => {
+    if (open) {
+      document.body.style.overflow = 'hidden';
+    } else {
+      document.body.style.overflow = '';
+    }
+    return () => { document.body.style.overflow = ''; };
+  }, [open]);
+
+  if (!open) return null;
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center">
+      <div className="absolute inset-0 bg-black/50" onClick={onClose} data-testid="modal-overlay" />
+      <div className="relative bg-surface rounded-xl shadow-lg max-w-md w-full mx-4 p-6" data-testid="modal-content">
+        {title && <h2 className="text-lg font-semibold mb-4">{title}</h2>}
+        {children}
+        <button
+          onClick={onClose}
+          className="absolute top-4 right-4 text-text-muted hover:text-text"
+          aria-label="Close"
+        >
+          <svg xmlns="http://www.w3.org/2000/svg" className="h-6 w-6" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M6 18L18 6M6 6l12 12" />
+          </svg>
+        </button>
+      </div>
+    </div>
+  );
+}