Fix "unknown blob" Docker push failure in build-and-push CI

[Copilot]

The build-and-push job was failing when pushing the Web image to GHCR with unknown blob. Manual docker build + docker push causes GHCR to fail resolving shared base layers (mcr.microsoft.com/dotnet/aspnet:10.0) across repository namespaces (celleseum-apiservice → celleseum-web) when they're pushed as separate steps.

Changes

• Added docker/setup-buildx-action@v3 — enables Buildx with proper cross-repo blob mounting support
• Replaced manual docker build + docker push with docker/build-push-action@v6 for both ApiService and Web — builds and pushes in a single step, eliminating the manifest/blob race condition

# Before
- name: Build and push Web
  run: |
    docker build -f Celleseum.Web/Dockerfile -t ghcr.io/$OWNER_LC/celleseum-web:latest -t ghcr.io/$OWNER_LC/celleseum-web:${{ github.sha }} .
    docker push ghcr.io/$OWNER_LC/celleseum-web:latest
    docker push ghcr.io/$OWNER_LC/celleseum-web:${{ github.sha }}

# After
- name: Build and push Web
  uses: docker/build-push-action@v6
  with:
    context: .
    file: Celleseum.Web/Dockerfile
    push: true
    tags: |
      ghcr.io/${{ env.OWNER_LC }}/celleseum-web:latest
      ghcr.io/${{ env.OWNER_LC }}/celleseum-web:${{ github.sha }}