Add contributor profile stats dashboard

[l8888888]

Closes #836

Summary

• Add key contributor stats to the profile dashboard: total earned, completed bounties, contributions, and streak.
• Add a GitHub activity chart using public GitHub events for commits, PRs, and issues.
• Add earning history chart and shared frontend utility modules required for a clean build.

Validation

• npm run build from frontend/

The GitHub activity integration uses the public events API and gracefully falls back to an empty 14-day chart if the API is unavailable.