Fix Assets publish job on default branch pushes

[ohmyfelix]

Summary

• fix the default-branch Assets pipeline by preventing the publish job from running on regular push events
• keep package publishing available for manual runs via workflow_dispatch
• this avoids failing master pipelines when npm publish credentials are missing/invalid

Root cause

The publish job was triggered on every push to master and executed npm publish with secrets.NPM_TOKEN. Recent default-branch runs failed in that step with npm registry auth errors, causing the whole Assets workflow to fail.

Validation

• gh run view 21982516272 --repo contributte/datagrid --log-failed (shows failing npm publish auth step)
• npm install && npm run build