style: update difficulty chart colors to match standard LeetCode palette

[Yashaswini-K-P]

Description

Standardized the difficulty chart color scheme across the application to match standard LeetCode color palettes (#00b8a3 for Easy, #ffc01e for Medium, and #ff375f for Hard). This replaces mismatched retro/neon hex codes and ensures visual consistency across comparison graphs, leaderboard stats, and user profile breakdown charts.

Linked Issue

Fixes #362

Changes Made

• Updated difficulty bar chart dataset colors in frontend/js/leaderboard/compare.js.
• Updated aggregate statistics doughnut chart background and border colors in frontend/js/leaderboard/stats.js.
• Updated performance line graph and difficulty breakdown doughnut chart dataset colors in frontend/js/user/historical-graphs.js.

Type of Change

• Bug fix
• New feature
• UI/Visual update
• Documentation update
• Refactor

Testing

• Tested locally
• Tested on mobile viewport (if applicable)
• No console errors introduced

Checklist

• My code follows the project's coding style
• I have formatted my code locally by running npx prettier --write . before submitting
• I am submitting my PR from a dedicated feature/* branch, not the main branch
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have updated documentation if required
• I have linked the relevant issue

Screenshots / Screen Recording

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