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/Button.tsx

@@ -0,0 +1,37 @@
+import { ButtonHTMLAttributes, forwardRef } from 'react';
+
+type Variant = 'primary' | 'secondary' | 'danger' | 'ghost';
+
+interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: Variant;
+  loading?: boolean;
+}
+
+const variantClasses: Record<Variant, string> = {
+  primary: 'bg-primary text-white hover:bg-primary-hover focus:ring-primary',
+  secondary: 'bg-secondary text-white hover:bg-secondary/80 focus:ring-secondary',
+  danger: 'bg-error text-white hover:bg-error/80 focus:ring-error',
+  ghost: 'bg-transparent text-primary hover:bg-primary/10 focus:ring-primary',
+};
+
+export const Button = forwardRef<HTMLButtonElement, ButtonProps>(
+  ({ variant = 'primary', loading = false, children, className = '', disabled, ...props }, ref) => {
+    return (
+      <button
+        ref={ref}
+        className={`inline-flex items-center justify-center px-4 py-2 rounded-lg font-medium transition-colors focus:outline-none focus:ring-2 focus:ring-offset-2 disabled:opacity-50 disabled:cursor-not-allowed ${variantClasses[variant]} ${className}`}
+        disabled={disabled || loading}
+        {...props}
+      >
+        {loading ? (
+          <svg className="animate-spin h-5 w-5 mr-2" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24">
+            <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4" />
+            <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z" />
+          </svg>
+        ) : null}
+        {children}
+      </button>
+    );
+  }
+);
+Button.displayName = 'Button';