Normalise API image response and expose selected image candidates in VM

[zigzagdev]

Description:

The API may provide images as either images[] or image_url (single).
Update mapping logic so the frontend always receives consistent image data for rendering.

###Acceptance Criteria:

• toWorldHeritageDetailVm returns images: WorldHeritageImageVm[] consistently.
• If images[] is empty but image_url exists, convert it into a single-element array.
• Sort images by sort_order.
• Ensure alt falls back to vm.title.