feat: add per-repository commit heatmap drawer

[VIDYANKSHINI]

Summary

This PR implements the per-repository commit heatmap functionality inside the Top Repos widget. Clicking a repository name now opens a drawer with a 90-day mini contribution heatmap and relevant metrics.

Closes #943

Type of Change

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

Changes Made

• Created RepoActivityDrawer component for displaying the per-repo heatmap and metrics (Escape to close, focus-trap logic).
• Created /api/metrics/repos/[...repo]/commits API endpoint to fetch up to 100 commits from the last 90 days.
• Updated TopRepos widget to make repository names clickable and open the drawer.
• Added a new external link icon next to repo names to preserve the ability to quickly open the repo in GitHub.

How to Test

Steps for the reviewer to verify this works:

1. Go to the dashboard and ensure the Top Repos widget loads.
2. Click on a repository name.
3. Verify the drawer opens from the right and displays the total commits, most active day, peak hour, and the 90-day mini heatmap correctly.
4. Verify pressing Escape closes the drawer.
5. Click the external link icon to verify it opens the repository on GitHub.

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

[vercel]

@VIDYANKSHINI is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus