Fix session file URLs (add /api prefix) and WebGL error handling in STL viewer ...

Backend:
- _file_url in content_store.py now returns /api/sessions/... instead of /sessions/...
- share_file.py URL construction updated to include /api
- content_publish.py docstring updated to reflect correct endpoint

Frontend:
- contentLinks.js: avoid double /api prefix in dev mode when backend already returns it
- stl.html: replace throw err with return after showing WebGL fallback message
- Rebuild dist

Tests:
- Update expected URLs in test_content_store.py and test_share_file.py

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Improve content publishing UX