feat: add stale PR indicator

[Saniya30pawar]

Summary

Added a stale PR indicator to PRMetrics so maintainers can quickly identify open pull requests that have been waiting too long for review.

Closes #257

Type of Change

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

Changes Made

• Added stale PR calculation based on PR creation date.
• PRs open for more than the selected threshold are counted as stale.
• Added configurable stale thresholds: 7, 14, and 30 days.
• Added an orange stale PR warning tile in the PR Analytics section.
• Added a GitHub search link for the user’s open stale PRs.
• Updated API response to include stale PR count, threshold, and search URL.

How to Test

Steps for the reviewer to verify this works:

1. Run the app locally.
2. Sign in with GitHub.
3. Open the dashboard.
4. Go to the PR Analytics section.
5. Check that the stale PR tile appears.
6. Change the Stale after dropdown between 7, 14, and 30 days.
7. Confirm the stale PR count updates based on the selected threshold.
8. Click the stale PR tile and verify it opens GitHub search for stale open PRs.

Checks

• npm.cmd run type-check
• npm.cmd run lint

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[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

[github-actions]

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[Priyanshu-byte-coder]

PR is currently conflicting with main. Please rebase onto main and resolve conflicts before this can be reviewed/merged:

git fetch upstream
git rebase upstream/main
# resolve conflicts
git push --force-with-lease