Wires gallery navigation on a rendered side panel: thumbnail clicks swap the
main image and move the active marker, and clicking the main image opens the
lightbox over the whole gallery with two-way synchronization back to the
thumbnails. Guarded by a one-shot _galleryEventsAttached flag so repeated
calls on the same panel are no-ops.
Parameters
sidePanelElement: HTMLElement|null
The side-panel root containing the gallery, or null.
lightboxManager: Lightbox|null
The lightbox to open on main-image click, or null.
Wires gallery navigation on a rendered side panel: thumbnail clicks swap the main image and move the active marker, and clicking the main image opens the lightbox over the whole gallery with two-way synchronization back to the thumbnails. Guarded by a one-shot
_galleryEventsAttachedflag so repeated calls on the same panel are no-ops.