Node 22+ everywhere, LF text on Windows checkouts, CI actions v7

- Node minimum is 22 in package.json engines, the start scripts, the service installer, setup and doctor,
  matching the README, the release notes and the CI matrix (22 / 24).
- .gitattributes: CSS, HTML, JSON, Markdown and YAML check out with LF, so `node --test` on Windows matches the
  CSS rules the UI tests read; the Windows test step is no longer continue-on-error.
- CI: actions/checkout@v7 and actions/setup-node@v7 (Node 24 runtime).

.gitattributes

@@ -8,6 +8,13 @@
 *.mjs text eol=lf
 *.js  text eol=lf
 *.py  text eol=lf
+# Text the tests read and match line by line (CSS rules, HTML, data, docs): LF everywhere, so a Windows checkout
+# (core.autocrlf=true) runs `node --test` exactly like Linux and macOS.
+*.css  text eol=lf
+*.html text eol=lf
+*.json text eol=lf
+*.md   text eol=lf
+*.yml  text eol=lf
 # binary assets
 *.png binary
 *.jpg binary