feat(gallery): add gallery block with grid/masonry layouts and lightbox

First-class image gallery block. Two layouts ship today - grid (uniform
tiles, configurable columns 1-6, aspect-ratio presets) and masonry
(CSS-columns Pinterest packing). Click-to-zoom lightbox built in Preact,
portalled to <body> so it escapes the section's `isolation: isolate`
stacking context; keyboard navigation (← → Esc), touch swipe, and body
scroll-lock included.

Two source modes: explicit items[] for curated galleries with per-image
captions/credits/links, or album: <folder> for drop-in folder loading
sorted by filename. Remote URLs (Unsplash/Pexels/CDNs) are passed
through unprocessed; local images get the full responsive variants +
inline base64 LQIP placeholder for instant first-paint over the eventual
sharp image.

Captions, titles, and credits are rendered through Hugo's RenderString
at build time (display: inline) - users get full Markdown (bold/italic/
code/links) in any of those fields with zero JS markdown parser. Alt
text falls back to plainified caption → humanised filename so screen
readers always have something useful.

Caption positions: below (default - accessibility-first), overlay
(magazine gradient), hover (clean grid reveal), none (lightbox-only).
Block-scoped CSS in the co-located style.css is picked up by the
existing block-CSS auto-discovery.

Test site: two demos under home.yaml (curated grid + auto-loaded
masonry) with 9 sample Picsum images stored under
test/assets/media/albums/gallery/ at varied aspect ratios.

test/config.yaml

@@ -1,4 +1,4 @@
-title: Hugo Blox Test
+title: HugoBlox Test
 baseURL: https://example.com
 build:
   buildStats:
@@ -14,11 +14,3 @@ markup:
   _merge: deep
 security:
   _merge: deep
-params:
-  header:
-    navbar:
-      enable: false
-  extensions:
-    cms:
-      local_backend: true
-      publish_mode: