f0397740d7
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>