Add stale pr indicators

[Mash4-4-4-4]

Summary

Added stale PR indicators in the PR Analytics section to help users quickly identify pull requests that have been open for too long.

Closes #257

Type of Change

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

Changes Made

• Added stale PR detection logic based on configurable thresholds
• Added threshold selector for 7 / 14 / 30 days
• Added stale PR count chip in the PR Analytics section
• Added stale PR list with orange "Stale" badges
• Added clickable links to open stale PRs directly
• Extended PR metrics API response to include PR metadata required for stale calculations

How to Test

Steps for the reviewer to verify this works:

1. Open the dashboard page
2. Navigate to the PR Analytics section
3. Verify stale PR count appears above the metrics cards
4. Change the threshold dropdown between 7 / 14 / 30 days
5. Confirm stale PR count updates correctly
6. Verify stale PRs display orange "Stale" badges
7. Click a stale PR entry and verify it opens the corresponding GitHub PR link

Screenshots (if UI change)

Added stale PR indicators, threshold selector, and stale PR list in the PR Analytics section.

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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