Implement interactive image gallery (thumbnail → hero image swap)

[zigzagdev]

Description:

Add an interactive gallery to the World Heritage detail page.
When a user clicks a thumbnail in the Gallery section, the hero image at the top of the page should update to the selected image. This improves study experience by allowing users to explore visual materials quickly.

Acceptance Criteria:

• Clicking a gallery thumbnail updates the hero image immediately.
• Selected thumbnail is visually highlighted.
• Primary image is selected by default on initial load.
• Works with page navigation (switching heritage ID resets selection).
• Graceful fallback when no images are available.

[zigzagdev]

Closing in favor of a different UX approach: instead of swapping the hero image, clicking a gallery thumbnail now opens the image enlarged in a Lightbox overlay with prev/next navigation (PR #390). Swapping the hero image turned out to conflict with the hero's existing layout/role (gradient overlay, title, responsive sizing tied to the primary image), so we pivoted to the more standard "click to enlarge" gallery pattern. Tracked in #391.