Improve onboarding REST fallback and errors

[shaypal5]

Summary

• add required-field validation for signup payloads
• wrap API routing in JSON error handling to avoid opaque worker failures
• document the exact REST signup payload and no-npx fallback
• add the REST fallback to the editable Adanim onboarding prompt
• cover incomplete signup payloads in tests

Verification

• npm run check
• npm run test
• npm run build