Add new power button feature

[Nazzoe]

Pull Request Summary

Title: Add exponentiation support to calculator (UI, API, and tests)

Summary:
This PR adds a new exponentiation feature to the calculator app. It includes:

• Front-end button and input handling for ^
• API support in api/controller.js for the power operation
• Unit tests for exponentiation in test/arithmetic.test.js

What changed

1. Added ^ button in public/index.html
2. In public/client.js, mapped ^ to operation=power and added keypress handling for ^
3. In api/controller.js, added 'power': function(a, b) { return Math.pow(a, b); }
4. Added exponentiation test cases in test/arithmetic.test.js
5. Added comment documentation in tests showing exponent function was added

Reason

• Extends core calculator functionality beyond add/sub/mul/div
• Demonstrates end-to-end Copilot-assisted feature implementation
• Adds regression coverage for exponent behavior

Closes #4