[Bug] `/leaderboard` route is linked from Dashboard but does not exist

[karastoyanov]

🐛 Bug Description

The Dashboard "Explore" section contains a card linking to /leaderboard, but no route, page component, or backend endpoint exists for it. Clicking the card silently fails — React Router renders nothing.

📋 Steps to Reproduce

1. Log in, go to /dashboard
2. Click the "Leaderboard" card in the Explore section

🎯 Expected Behavior

A leaderboard page loads.

📸 Actual Behavior

The URL changes to /leaderboard but the page is blank. No route is defined in App.jsx.

🌍 Environment

All environments.

📎 Additional Context

Defined in Dashboard.jsx:44 but absent from App.jsx. Related to the Leaderboard feature story.