Enhanced calculator with backspace, Glassmorphic UI, and Logic Improvements

[hasan-dhanish]

This PR improves the Calculator project by adding missing features and overhauling the UI.
Changes Made

• New Feature: Added a Backspace button (⌫).
• UI Overhaul: Implemented a modern Glassmorphism design using CSS Grid for better layout and responsiveness.
• Logic Improvement: Updated the compute() function to properly display calculation history vs. current result.
• Documentation: Fixed a typo in CONTRIBUTION.md.