- Add umbral_name to ProfileUpdate schema with regex validation
- Add uniqueness check in PUT /auth/profile handler
- Replace disabled input with editable save-on-blur field in Social card
- Client-side validation (3-50 chars, alphanumeric/hyphens/underscores)
- Inline error display for validation failures and taken names
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>