- manifest.json already present; add <link rel="manifest"> to help.html
so all Nexus HTML pages declare PWA support consistently
- add tests/test_manifest.py to verify manifest exists, is valid JSON,
has required PWA fields, and is referenced from both index.html and help.html
Fixes#832
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>