fix: dynamic contribution heatmap intensity scaling

[rAmAn678112]

Summary

Fixes incorrect contribution heatmap intensity scaling by replacing fixed thresholds with dynamic scaling based on the user's maximum commit count.

Closes #936

Type of Change

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

Changes Made

• Replaced fixed heatmap intensity thresholds with dynamic scaling
• Added relative contribution intensity calculation using max commit count
• Updated heatmap legend to reflect dynamic bucket scaling

How to Test

1. Run the application using npm run dev
2. Open the dashboard contribution heatmap
3. Compare days with low and high commit counts
4. Verify that higher contribution days display stronger intensity levels dynamically

Screenshots (if UI change)

N/A

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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[github-actions]

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