feat: add calculator UI, operations, and keyboard support

[Pramod-Munnoli]

✨ Description

This PR adds a fully functional calculator built using vanilla JavaScript, HTML, and CSS.
The calculator includes the complete user interface, core operations, and keyboard support as defined in the project’s acceptance criteria.

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✅ Features Implemented

• Responsive calculator UI built with CSS Grid
• Core operations: +, −, ×, ÷, %, +/-, AC, Backspace
• Decimal input validation (prevents multiple decimals)
• Handles division by zero gracefully (displays “Error”)
• Float rounding up to 10–12 significant digits
• Full keyboard support:
  • Digits 0–9
  • Operators +, −, *, /
  • Enter → equals
  • Escape → clear (AC)
  • Backspace → delete last digit
• ARIA labels and focus styles for accessibility
• Works as a single self-contained HTML file (no dependencies)

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🖼️ Screenshots

• The full calculator UI
• Error message on division by zero

Calculator UI	Error message on division by zero

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🧪 Testing

Manually tested locally in the browser:

• ✅ All buttons functional
• ✅ Keyboard inputs working
• ✅ Responsive layout on desktop and mobile
• ✅ Correct results with float precision and rounding
• ✅ Proper error handling for invalid operations

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🔗 Related Issue

Closes #161

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🗒️ Notes

• Built entirely using vanilla JS, HTML, and CSS (no frameworks or CDNs).
• The calculator can be opened directly in any browser — no build tools required.
• Open to suggestions for UI styling or additional improvements.

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🏷️ Hacktoberfest

This PR is submitted for Hacktoberfest 2025.
If the repository is not automatically marked as participating, kindly add the label hacktoberfest-accepted. 🙏

[vercel]

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