ci: verify frontend assets are included in source tarball

Add a check that dist/index.html exists in the tarball after
generation. Fails fast if the pre-built frontend wasn't included,
which would cause the desktop app to show "Connection refused".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -99,6 +99,8 @@ jobs:
             --exclude='*.src.rpm' \
             .
           mv /tmp/monsoon-${VERSION}.tar.gz .
+          # Verify frontend assets are in the tarball
+          tar tzf monsoon-${VERSION}.tar.gz | grep -q "dist/index.html" || { echo "ERROR: dist/index.html missing from tarball"; exit 1; }
 
       - name: Vendor Rust dependencies
         run: |