Make a brand kit the default

curl -X POST 'https://www.unitpost.com/api/v1/brand-kits/bk_123/set-default' \
  -H 'Authorization: Bearer YOUR_API_KEY' \
  -H 'User-Agent: my-app/1.0'

{
  "object": "brand_kit",
  "id": "id_123",
  "name": "Example",
  "is_default": false,
  "website_url": "https://example.com",
  "description": "string",
  "tone": "NEUTRAL",
  "voice_notes": "string",
  "industry": "string",
  "core_pages": [
    {
      "url": "https://example.com",
      "title": "string"
    }
  ],
  "extracted": {
    "colors": [
      {
        "hex": "string",
        "label": "string"
      }
    ],
    "logo_urls": [
      "string"
    ],
    "background_urls": [
      "string"
    ],
    "fonts": [
      "string"
    ]
  },
  "graphics": [
    {
      "library_image_id": "library_image_123",
      "image_id": "image_123",
      "path": "string",
      "role": "logo",
      "label": "string"
    }
  ],
  "source": "string",
  "updated_at": "2026-01-01T00:00:00.000Z"
}