Arena leaderboard: add checkbox to hide proprietary models

[juodumas]

trafficstars

Why are these changes needed?

I want to quickly find the best models with open weights.

Related issue number (if applicable)

Closes #3614

Checks

• [x] I've run format.sh to lint the changes in this PR.
• [ ] I've included any doc changes needed.
• [ ] I've made sure the relevant tests are passing (if applicable).