Fix: Add z-index to leaderboard container for improved stacking conte…

[ManavVaishnani]

Checklist:

• [*] I've thoroughly self-reviewed my changes
• [*] I've added tests for my changes, unless they affect admin-only areas (or are otherwise not worth testing)
• [*] I've verified any visual changes using Percy (add a commit with [percy] in the message to trigger)

Changes Made:

• Add z-index of z-10 to the leaderboard container

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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