fix: normalize quiz join inputs

[QvarcY]

Summary

Fixes #159.

The quiz join form now normalizes participant input before it is submitted:

• strips whitespace and other non-alphanumeric characters from pasted game PINs
• uppercases the PIN and applies the six-character limit after normalization
• trims nicknames on submit without preventing spaces inside names while typing
• rejects whitespace-only nicknames with a clear message
• uses the normalized nickname consistently for the API request, localStorage and navigation state
• adds regression coverage for pasted PINs and whitespace-only nicknames

Why

Previously the browser's maxLength={6} could count pasted whitespace toward the PIN length before the value was normalized. A valid copied code could therefore be truncated and produce a misleading "Quiz not found" error.

Nicknames were also submitted without trimming, and a whitespace-only nickname passed the existing truthiness check.

Verification

• targeted JoinQuiz tests: 2/2 passed
• full frontend test suite: 27/27 passed
• changed files pass ESLint
• frontend production build passes
• git diff --check passes

Existing lint baseline

The repository-wide frontend lint already fails on unrelated files in current main.

Before this change:

• 20 errors
• 2 warnings

After this change:

• 19 errors
• 2 warnings

The reduction comes from removing the existing any in JoinQuiz.tsx. Both files changed by this PR are lint-clean.

[TheCodeHeist-Coder]

Awesome work! 🔥
Thanks for taking up the issue and fixing it. This is a great first contribution to the project — really appreciate it!

Welcome aboard, and looking forward to more contributions from you.

[TheCodeHeist-Coder]

It's a clean and as expected code