Usernames are auto-assigned at signup (shipped earlier this session)
but there was still no way to change one. Adds a Username field to
Settings, validated client- and server-side against the same
3-20-char lowercase/digits/underscore pattern used for generation,
with a 409 on collision (excluding the user's own current username).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Arnaud
f0397740d7