feat(repos): add Contribution Health sidebar on Contributing tab

[JamesR111]

Summary

Adds a Contribution Health card to the repository details sidebar when the Contributing tab is active. The card surfaces four repo-level activity metrics with status badges (Great / Good / Fair / Low)

Metrics shown:

• PR Merge Rate — merged vs closed-unmerged PRs for the repo
• Issue Solve Rate — issues closed with a linked PR vs closed without
• Time to Merge — average days from PR open to merge
• Active Contributors — unique PR authors in the last 35 days

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Screenshots

Contributing tab — sidebar with Contribution Health

Checklist

• New components are modularized/separated where sensible
• Uses predefined theme (e.g. no hardcoded colors)
• Responsive/mobile checked
• Tested against the test API
• npm run format and npm run lint:fix have been run
• npm run build passes
• Screenshots included for any UI/visual changes