Fix Pages build job to use the actual frontend project paths

[Copilot]

The GitHub Actions build job failed before install/build because setup-node was configured with a non-existent lockfile path under webapp. This PR aligns the workflow with the real Next.js project location in app.

• Root cause addressed

  • cache-dependency-path referenced webapp/package-lock.json, which is absent.
  • Job default working directory and artifact upload path were also pointed at webapp, while the built frontend lives in app.

• Workflow path alignment

  • Updated build job working-directory from webapp to app.
  • Updated npm cache lockfile path from webapp/package-lock.json to app/package-lock.json.
  • Updated Pages artifact path from webapp/out to app/out.

• Representative diff

  defaults:
    run:
      working-directory: app
  
  - uses: actions/setup-node@v4
    with:
      node-version: 20
      cache: npm
      cache-dependency-path: app/package-lock.json
  
  - uses: actions/upload-pages-artifact@v3
    with:
      path: app/out