Add username suggestion when taken

[hyattc1]

Generated by cursor:
Implements feature request for username suggestions when username is taken.

 **Changes:**
 - Modified `public/src/client/register.js` to suggest username + '1' when original username is taken
 - Shows English-only message "This username is taken. Try '[username]1'" instead of i18n error
 - Follows prompt requirements for English-only implementation
 
 **Testing:**
 - Try registering with an existing username
 - Should now see suggestion instead of generic error message