📊 Add GitHub-style heatmap showing tasks completed per day

[anoopcodehack]

File: client/src/pages/Dashboard.jsx

GitHub has that cool green heatmap showing activity.
DevBoard should have one too — showing how many
tasks you moved to Done each day!

Fix:

• tasks have createdAt from MongoDB ✅
• Group done tasks by date
• Render a 7×12 grid with opacity based on count
• Hover shows "3 tasks completed on June 3"

No package needed — pure CSS grid + JS!
This will make people SCREENSHOT and share the app 🔥