feat: add real-time activities tracked stat card

[RaghavGanesh7]

🌱 Carbon Footprint Tracker - Pull Request

Description

• Add subscribeToUserSubmissionCount function in firestore.ts for real-time count
• Add new stat card showing total activities tracked with user join date tooltip
• Update dashboard grid from 3 to 4 columns for new card
• Add tooltip support to StatCard component

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🎨 Style/UI update
• ♻️ Code refactoring
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issue

Fixes #16

Changes Made

Screenshots (if applicable)

Testing

• I have tested this change locally
• I have tested on mobile devices
• I have tested on different browsers
• All existing tests pass
• I have added new tests (if applicable)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

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For Maintainers

• Code review completed
• Tests passing
• Documentation updated
• Ready to merge

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