fix: improve drag overlay behavior in routine builder

[KrithikaVellingiri]

📌 Description

This PR improves the drag-and-drop experience in the Routine Builder by fixing visual layering and interaction issues during task scheduling.

Previously, the dragged task card appeared behind the weekly schedule grid while dragging, which caused poor visual feedback. Additionally, after dropping a task into the schedule, the dragged card visually snapped back into the Task Library even though the scheduling action had already succeeded, making the interaction feel confusing and inconsistent.

This implementation uses DragOverlay from @dnd-kit/core to create a separate overlay-based drag layer, improving drag visibility and eliminating the confusing snap-back behavior.

🔗 Related Issue

Closes #319

🛠 Changes Made

• Fixed dragged task cards rendering behind the weekly schedule grid
• Prevented the original draggable element from visually snapping back after drop
• Added DragOverlay for overlay-only drag interactions

📸 Screenshots (if applicable)

before.vs.after.mp4

✅ Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue