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/app/(auth)/layout.tsx

@@ -0,0 +1,16 @@
+import { I18nProvider } from '@/lib/i18n';
+import { ToastProvider } from '@/components/ui/Toast';
+
+export default function AuthLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  return (
+    <I18nProvider initialLocale="de">
+      <ToastProvider>
+        {children}
+      </ToastProvider>
+    </I18nProvider>
+  );
+}