Fix Deploy workflow failure caused by npm peer dependency resolution

[Copilot]

Recent Deploy runs started failing during dependency install: the Astro deploy action path executes npm install, which now fails with ERESOLVE on the @astrojs/check / TypeScript 6 peer constraint. This updates the deploy build path to use the same install strategy already used in CI.

• Workflow change: replace withastro/action with explicit build steps

  • Setup Node 24 via actions/setup-node
  • Install dependencies with npm ci --legacy-peer-deps
  • Build with npm run build
  • Upload dist via actions/upload-pages-artifact

• Behavioral impact

  • Deploy now uses lockfile-based installs with legacy peer handling, avoiding the peer-resolution failure introduced by plain npm install.
  • Artifact upload/deploy flow remains the same (deploy-pages job unchanged).

- name: Install dependencies
  run: npm ci --legacy-peer-deps

- name: Build site
  run: npm run build

- name: Upload Pages artifact
  uses: actions/upload-pages-artifact@v5
  with:
    path: ./dist