Add username validation rules #141
Description
Priority: Tier 3 — Medium
No username format enforcement. Users can set any string as username including spaces, special chars, or reserved names.
Acceptance Criteria
- Enforce: 3-20 characters, alphanumeric + underscores only
- Case-insensitive uniqueness check
- Block reserved names (admin, moderator, system, api, etc.)
- Validation on both client and server side
- Error message explaining format requirements