feat: add 7 utility shortcodes (admonition, button, carousel, gallery, figure, tabs, video)

[adurrr]

Resolves #6

What's included

7 utility shortcodes + 1 CSS file + code-copy.js enhancement:

Shortcode	Features
admonition	4 variants (note/tip/warning/danger), optional title, markdown content, Feather SVG icons
button	3 variants (primary/secondary/outline), icon slot, auto target=_blank for external
carousel	CSS scroll-snap with minimal JS prev/next buttons
figure	HTML5 <figure> with caption, lazy loading, local+remote resources
gallery	CSS grid thumbnails, optional :target CSS-only lightbox
tabs / tab	CSS-only radio-button tabs, keyboard-accessible (Tab + arrow keys)
video	HTML5 <video> with poster, caption, responsive container

Design decisions

• wvc- CSS prefix to avoid theme conflicts
• Dark-mode aware via [data-theme] + CSS variables
• Single assets/css/shortcodes.css for all styles (346 lines)
• Feather-style open-source SVG icons (MIT licensed)
• Theme-aware code block backgrounds (slate in both light/dark modes)
• Copy-to-clipboard button on ALL code blocks (highlight + plain pre)

Code copy improvements

• assets/js/code-copy.js now targets both .highlight and plain <pre> blocks
• Copy button redesigned: semi-transparent dark pill, white icon, 32x32px
• Works on all code blocks regardless of language specifier

Tests

• 18 new Hugo integration tests added (34 total, all passing)
• 91 JS unit tests still pass (no regression)
• Example site builds with 80 pages including demo at /docs/shortcodes/

Files

assets/css/shortcodes.css                    (new - 346 lines)
assets/js/code-copy.js                       (added - was missing on main)
layouts/_shortcodes/{admonition,button,carousel,figure,gallery,tab,tabs,video}.html
exampleSite/layouts/_default/baseof.html     (CSS + code-copy load)
exampleSite/content/docs/shortcodes.md       (demo page)
tests/hugo/shortcode_utils_test.go           (18 tests)
tests/hugo/testdata/{18 fixtures}