feat: add contributor profile stats dashboard (Bounty #836)

[liqiuniu]

Summary

Contributor Profile Stats Dashboard with GitHub activity integration. Bounty #836

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Features

1. GitHub Activity Graph

• Fetches user's public events via GitHub API
• Shows commits, PRs, issues over last 30 days
• Area chart with gradient fill

2. Earnings History Chart

• Bar chart showing FNDRY payouts over time
• Total earnings summary

3. Key Stats

• Total Earned - All FNDRY received
• Bounties Completed - Count of finished bounties
• Current Streak - Consecutive days with activity
• Longest Streak - Best streak record

4. Overview Tab

• Stats grid with 4 key metrics
• Activity chart + Earnings chart side by side
• Contribution summary (commits/PRs/issues)

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Files

• frontend/src/api/github.ts - GitHub API + earnings API
• frontend/src/hooks/useGitHubActivity.ts - React hooks for data
• frontend/src/components/profile/ActivityCharts.tsx - Chart components
• frontend/src/components/profile/ProfileDashboard.tsx - Enhanced dashboard

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Acceptance Criteria

• GitHub activity graph (commits, PRs, issues)
• Earnings history chart
• Key stats: total earned, bounties completed, streak

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Closes #836

Wallet: 7xKXtg2CW87d97TXJSDpbD5jBkheTqA83TZRuJosgAsU