Suggesting alternative username when taken (client-side)

[mhk2-cmu]

Summary

When a username is already taken during registration, show a friendly suggestion based on the user’s input (append suffix), per Issue #1.

What changed

• public/src/client/register.js: replace the plain “username taken” error with:
  • read current input value
  • compute suggestion = <entered> + "suffix"
  • show: Username taken. Maybe try "<suggestion>"

Why

Improves UX by pointing the user to an alternative immediately (as requested in the issue).

How to test

1. Start the app and open /register.
2. Type a username you know is taken (e.g., test123).
3. Blur the field or try to submit.
4. Expected: inline message Username taken. Maybe try "test123suffix".

Notes

• Frontend-only, minimal change.
• No new deps, no backend changes.

Fixes #1