feat(readme): add real image to replace placeholder demo

[naheel0]

📸 Feature Request

Replace the current placeholder image in the README demo section with a real screenshot of the application.

🧩 Current State

The README currently uses a placeholder image:
image

🚀 Proposed Change

• Capture a real screenshot of the app (e.g., Discover page, Dashboard, or Leaderboard)
• Add the image to the root-level /assets folder
• Update the README to use the new image

📁 File Location

/assets/gitquest-demo.png

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✅ Acceptance Criteria

• Placeholder image is removed
• Real screenshot is added to /assets
• README updated with correct image path
• Image renders properly on GitHub

💡 Notes

• Use a clear, high-quality screenshot
• Prefer a UI that best represents the core experience (quest discovery is ideal)
• Optimize image size for fast loading

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🙌 Good First Issue

Perfect for first-time contributors!