feat(gallery): detail-page navigation, code ergonomics, image polish (#300)

Detail pages were dead ends: a reader who finished one example had to go
back to the grid to find the next. They now page within their own kind
(prev/next, arrow keys), list their tags as links back to the filtered
index, and end with three related entries chosen by shared topic tags.

Source listings gain a line-number gutter and collapse past 120 lines;
every code block gets a copy button with a non-secure-context fallback
and a visible failure state; README headings get stable anchors.

Card images carry intrinsic dimensions so the grid no longer shifts as
they load, and the lightbox only closes on backdrop, button or Esc --
clicking the image toggles native size instead.

Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: TMHSDigital