feat: Implement a side-by-side user comparison tool for the leaderboard

[Vachhani-Tapan]

Feature Description

Add a comparison feature to the leaderboard that allows users to select up to 3 peers and compare their LeetCode stats and progress side-by-side using interactive charts and statistics. This will boost engagement by encouraging healthy competition and allowing students to analyze their grinding velocity compared to their peers.

User Flow

1. Selection: Add a checkbox next to each student's name on both desktop (leaderboard-row) and mobile (mobile-card) views.
2. Action: Once 2 or 3 users are selected, a floating bar appears at the bottom with a "Compare" button.
3. Comparison Screen: Clicking "Compare" opens a modal or dedicated panel displaying:
   • Metrics Comparison Table: Side-by-side table comparing score, easy/medium/hard problems solved, and average daily problems.
   • Difficulty Breakdown Chart: A comparative bar chart (Easy/Medium/Hard) for each user.
   • Progress History Chart: A combined line graph showing cumulative problems solved over time (using the /api/student/:username history endpoint).

Proposed Changes

1. frontend/leaderboard.html: Add checkboxes to row elements, a floating action panel, and a comparison overlay/modal that matches the existing dark/matrix style.
2. frontend/js/compare.js (New File): Handle selection state, fetch history data for compared users from /api/student/:username, and render comparison charts using a lightweight graphing library (like Chart.js via CDN).

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[Vachhani-Tapan]

@jagdish-15 Sir I would like to work on this Issue Please Assign me this for GSSOC 26

[jagdish-15]

This seems like a fair feature addition overall.

One UI suggestion though: instead of always showing checkboxes beside every user entry, I think it would fit the current interface better if comparison mode had to be explicitly enabled first (for example via a "Compare Users" button/toggle). That way the leaderboard UI stays cleaner during normal usage.

The rest of the proposed flow and logic sounds reasonable to me. Also, please make sure the UI remains consistent with the existing terminal-style theme of the platform across the comparison modal, charts, buttons, and mobile layout as well.