add power button

[akaz05p]

This pull request introduces a new "power" operation (x^y) to the calculator application, enabling users to perform exponentiation. The changes span the backend, frontend, and testing layers to ensure full support for the new feature.

Backend Changes:

• Added a new power operation to the operations object in api/controller.js, using Math.pow to calculate exponentiation.

Frontend Changes:

• Updated public/client.js to handle the ^ operator in the calculate function and keypress event listener. [1] [2]
• Added a new button for the x^y operation in the calculator UI (public/index.html).

Testing Changes:

• Added a comprehensive suite of tests for the power operation in test/arithmetic.test.js, covering various scenarios such as positive/negative powers, zero, and floating-point numbers.

Closes #1