Check out Git LFS files in frontend test CI

The QTI viewer's items.js fixture is stored via Git LFS and is now
imported by frontend test specs, so the frontend-tests workflow
needs to resolve LFS content on checkout rather than pulling the
pointer file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rtibbles

.github/workflows/frontend-tests.yml

@@ -30,6 +30,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v6
+      with:
+        lfs: true
     - name: Use Node.js
       uses: actions/setup-node@v6
       with: