fix: make web artifact download optional in Mac CI

The Mac CI was failing when only Mac files changed because it tried to
download web build artifacts that were never created (Node.js CI was skipped).

Added continue-on-error to the artifact download step, allowing the Mac build
to proceed and build the web frontend itself via the existing build-web-frontend.sh
script that's already part of the Xcode build process.

This fixes CI failures like the one in PR #153 where Mac-only changes
would fail due to missing web artifacts.

.github/workflows/mac.yml

@@ -140,6 +140,7 @@ jobs:
     
     - name: Download web build artifacts
       uses: actions/download-artifact@v4
+      continue-on-error: true
       with:
         name: web-build-${{ github.sha }}
         path: web/