feat: Replace the gulp UI toolchain with Vite

Vite is what stackable-apps and stackable-cockpit build with, and the
gulp pipeline was frozen: gulp-eslint/gulp-stylelint are unmaintained
and pinned eslint 6 / stylelint 13. Upstream antora-ui-default still
ships the same frozen toolchain, so there is no upstream modernisation
to converge with.

- build.mjs produces the identical bundle layout: js entries are built
  as self-contained classic scripts (one vite pass each), the css in a
  separate pass, static directories are copied, everything is zipped to
  build/ui-bundle.zip. Verified by building the full site with both
  toolchains: every page is byte-identical.
- The vendor bundle sources become ESM and reference the browser build
  of @asciidoctor/tabs explicitly (vite does not honor the style/browser
  package fields the way browserify did).
- CSS custom properties are no longer inlined at build time; every
  supported browser handles var(), and the upcoming design-token work
  needs them preserved.
- The standalone gulp preview is gone; render UI changes with real
  content via 'make build-truly-local' or the deploy preview.
- eslint 9 (flat config) and stylelint 16; the stylelint config accepts
  the inherited upstream css conventions instead of reformatting
  everything right before the planned upstream re-sync.
- Node 24 in .nvmrc/engines/CI, matching stackable-apps.

Author: lfrancke

.github/workflows/ui.yml

@@ -14,7 +14,8 @@ jobs:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - uses: actions/setup-node@cdca7365b2dadb8aad0a33bc7601856ffabcc48e # v4.3.0
         with:
-          node-version: '16'
+          node-version: '24'
           cache: npm
       - run: npm ci
+      - run: npm run lint --workspace ui
       - run: npm run bundle --workspace ui

.nvmrc

@@ -0,0 +1 @@
+24

Makefile

@@ -40,7 +40,7 @@ build-prod: build-ui
 	node_modules/.bin/antora generate antora-playbook.yml --fetch $(ANTORAFLAGS)
 
 build-ui:
-	node_modules/.bin/gulp --cwd ui bundle
+	npm run bundle --workspace ui
 
 build-search-index:
 	npm run build-search-index

README.adoc

@@ -62,7 +62,7 @@ content:
 
 The design & layout lives in the `ui/` directory of this repository.
 
-`LIVERELOAD=true gulp` may be used to recreate the built documentation after each edit.
+To see UI changes rendered with real content, rebuild with `make build-truly-local`.
 
 == More useful links