Implement power function and UI changes

[stargazer4]

Implements the power function in the calculator application, including backend logic, frontend UI updates, and instructional content changes.

• Backend Implementation: Adds a power function to the operations object in api/controller.js, utilizing Math.pow(a, b) for calculation.
• Frontend UI Updates: Introduces a new button for the power operation in public/index.html and updates public/client.js to handle the power operation in the calculate function's switch statement.
• Instructional Content Changes: Updates .instructions/2. core exercises.md and .instructions/3. challenge exercises.md to include instructions and challenges related to implementing and testing the power function.

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For more details, open the Copilot Workspace session.