feat: Add dark/light mode toggle in navbar

[Shweta0416]

📋 PR Checklist

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🔗 Related Issue

Closes #

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📝 What does this PR do?

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🗂️ Type of Change

• 🐛 Bug fix
• [ X ] ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Ran the backend locally (uvicorn app.main:app --reload)
• Ran the frontend locally (npm run dev inside frontend/)
• Tested the affected API endpoints manually
• Added / updated tests

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📸 Screenshots (if UI change)

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⚠️ Anything to flag for reviewers?

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed