feat(ui): Add custom slim scrollbar styling to dashboard widgets

[rahul-rajak-nsut]

Summary

Adds custom slim scrollbar styling to all scrollable dashboard widgets using tailwind-scrollbar v3.1.0. Replaces the bulky OS-default scrollbar with a clean 6px pill-shaped thumb that works in both light and dark mode.
Closes #585

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Installed tailwind-scrollbar v3.1.0 (compatible with Tailwind v3)
• Added .scrollbar-thin utility in globals.css with dark mode support
• Applied scrollbar-thin to BadgeSection.tsx, ContributionHeatmap.tsx, FriendComparison.tsx, NotificationBell.tsx

How to Test

1. Run npm run dev and open http://localhost:3000
2. Scroll inside any dashboard widget (Contribution Heatmap, Friend Comparison, Notifications)
3. Verify scrollbar appears slim (6px) with rounded thumb
4. Toggle dark mode and verify thumb color adjusts

Screenshots (if UI change)

Slim scrollbar visible on all scrollable widgets in both light and dark mode

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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