Add light/dark theme toggle and new operators (power and square root)

[Copilot]

Overview

This PR implements the UI improvements and new functionality requested in #[issue_number], adding a manual theme toggle and two new mathematical operators to enhance the calculator's capabilities.

Changes Made

🌓 Light/Dark Theme Toggle

Added a manual theme toggle button that allows users to switch between light and dark modes:

• Toggle button with 🌓 icon positioned in the top-right corner of the calculator
• Theme preference persists across sessions using localStorage
• Smooth CSS transitions between themes
• Works alongside existing automatic system preference detection
• Fully accessible with proper ARIA labels

Light Mode:

Dark Mode:

🔢 New Mathematical Operators

Power Operator (^)

• Raises a number to a specified power
• Example: 3^2 = 9, 2^-2 = 0.25
• Keyboard shortcut: ^ key
• Backend endpoint: /arithmetic?operation=power&operand1=3&operand2=2

Square Root Operator (√)

• Calculates the square root of a number
• Example: √9 = 3, √2.25 = 1.5
• Backend endpoint: /arithmetic?operation=sqrt&operand1=9&operand2=0

Both operators are accessible via on-screen buttons and keyboard input, maintaining the calculator's existing interaction patterns.

Technical Details

Files Modified

• api/controller.js: Added power and sqrt operations to the backend controller
• public/client.js: Implemented theme toggle logic and handlers for new operators
• public/default.css: Added theme toggle button styling and enhanced theme switching support
• public/index.html: Added theme toggle button and new operator buttons to the UI
• test/arithmetic.test.js: Added 7 new test cases covering both operations

Code Quality

• Fixed existing ESLint errors (unnecessary escape characters in regex patterns)
• All 31 tests passing (24 existing + 7 new)
• Maintained 86%+ code coverage
• Zero linting errors

Accessibility Enhancements

• Theme toggle button includes proper ARIA labels
• New operator buttons have descriptive ARIA labels
• Keyboard navigation fully supported for new operators
• High contrast maintained in both themes

Testing

The implementation includes comprehensive test coverage:

• 4 tests for power operation (positive power, power of 0, negative power, negative base)
• 3 tests for square root operation (positive integer, decimal, zero)
• All existing tests continue to pass

Example test cases:

// Power: 2^3 = 8
request.get('/arithmetic?operation=power&operand1=2&operand2=3')
  .expect(200, { result: 8 })

// Square root: √9 = 3
request.get('/arithmetic?operation=sqrt&operand1=9&operand2=0')
  .expect(200, { result: 3 })

Breaking Changes

None. This PR only adds new functionality without modifying existing behavior.

Related Issues

Closes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update the UI and accesbility features</issue_title>
<issue_description>I want to make the UI better. Add light dark theme.
Add a few operators like square rooot and power of.

</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #20

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