feat: Replace the gulp UI toolchain with Vite

[lfrancke]

Description

Phase 2a of the docs overhaul, stacked on #873 (GitHub retargets this to main when #873 merges). Five commits, reviewable independently - happy to split into separate PRs if preferred:

1. chore: Commit the minified images - gulp-imagemin ran on every build; the minified results are committed instead (-673 lines of SVG cruft).
2. chore: Remove upstream leftovers from the UI - .gitlab-ci.yml and upstream's own docs/ component, never used here.
3. chore: Add a NOTICE file - minification strips embedded license header comments from the built css/js, so the third-party attributions (Font Awesome CC BY 4.0/OFL/MIT, highlight.js BSD-3, Asciidoctor Tabs MIT, Octicons MIT, the OFL fonts, MPL-2.0 upstream fork) live in ui/NOTICE, which the build ships in the bundle and therefore publishes on the site. The Octicons attribution inside the SVG's <desc> is preserved.
4. feat: Replace the gulp UI toolchain with Vite - the main course, see below.
5. fix: Remove invalid css declarations and lint leftovers - background-color: none/min-width: none are invalid and were ignored by browsers; removing them changes nothing.

The toolchain swap

• Why Vite: it's what stackable-apps and stackable-cockpit use, and the gulp pipeline was frozen (gulp-eslint/gulp-stylelint unmaintained, eslint 6, stylelint 13). Upstream antora-ui-default ships the same frozen toolchain, so there's no upstream modernisation we'd diverge from. Everything starts on the current majors: Vite 8, eslint 10, stylelint 17, archiver 8, Node 24.
• ui/build.mjs produces the identical bundle layout (js/, js/vendor/, css/, font/, webfonts/, img/, helpers/, layouts/, partials/, NOTICE, LICENSE → build/ui-bundle.zip). JS entries are built as self-contained classic scripts, one Vite pass each - rollup cannot code-split IIFE output, which is exactly what <script src> consumers need.
• The two vendor bundle sources are now ESM and reference @asciidoctor/tabs' browser build and css explicitly (Vite doesn't honor the style/browser package fields the way browserify/postcss-import did).
• CSS custom properties are no longer inlined at build time - every supported browser handles var(), and the planned design-token work needs them preserved. This is the only intentional CSS output change.
• The standalone gulp preview is gone; UI changes render with real content via make build-truly-local or the deploy preview.
• stylelint is configured to accept the inherited upstream CSS conventions rather than reformatting everything right before the planned upstream re-sync.
• package-lock.json had to be regenerated; @antora/ and pagefind are pinned to their previously locked versions* (3.1.7 / 1.3.0) so the rendered site and search assets stay identical - bumping those is Renovate's job, not this PR's.
• The root .nvmrc raises the Netlify build to Node 24.

Verification

Full site built with the gulp bundle and the Vite bundle, every page compared: 0 differing pages out of 220 (after normalising the trailing whitespace that #873's lint commit removed from an inline script). Bundle file lists are identical except the standalone css/vendor/fontawesome.css copy, which nothing referenced (Font Awesome stays inlined in site.css - its removal is a separate, planned performance PR). npm run lint --workspace ui and make build-ui pass; the UI CI workflow now also runs the lint.