Modernize calculator UI with theme switching functionality

[Copilot]

UI Modernization and Theme Switching Implementation

Successfully implemented modern UI design and theme switching functionality for the calculator.

Completed Work:

• Fix ESLint configuration issue (removed invalid "request" environment)
• Design and implement modern CSS styling with glassmorphism effects
• Create 4 theme options (default gradient, dark, light, colorful)
• Add theme toggle icon (🌓) with smooth animations and hover effects
• Implement theme persistence with localStorage
• Test all calculator operations still work correctly
• Test theme switching functionality works seamlessly
• Verify all tests pass and linting is clean

Key Features:

• Modern Design: Beautiful gradient backgrounds, glassmorphism effects, rounded corners, and smooth transitions
• Multiple Themes: 4 distinct themes that users can cycle through
• Theme Persistence: Selected theme is remembered across browser sessions
• Responsive Design: Optimized for different screen sizes
• Maintained Functionality: All arithmetic operations work perfectly (tested addition, multiplication, division)

Before vs After:

• Before: Vintage calculator with blue background and basic gray buttons (Original UI)
• After: Modern calculator with multiple beautiful themes:
  • Default Theme - Purple gradient with glassmorphism
  • Dark Theme - Sleek dark design
  • Light Theme - Clean light design
  • Colorful Theme - Vibrant orange-pink gradient

Technical Changes:

• Enhanced CSS with CSS custom properties for theming
• Added theme management JavaScript with localStorage integration
• Improved button interactions with hover and active states
• Modern typography and spacing improvements
• Fixed ESLint configuration and code quality issues

All tests passing ✅ | Linting clean ✅ | All calculator operations verified ✅

Fixes #1.

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[haslam93]

Actually can you also add a new opetaro called Modulo.

[Copilot]

Actually can you also add a new opetaro called Modulo.

Added modulo operator (%) functionality! The calculator now supports modulo operations with:

• New % button in the UI
• Keyboard support (press % key)
• Full backend API implementation
• Comprehensive test coverage including edge cases

All tests pass and the operator works correctly. (2a1a9f2)