feat(calculator): Implement 'multiply' functionality

[aido-mth]

Description

This PR implements the multiplication logic in the calculator. It enables the previously disabled multiplication button in the UI and handles the calculation in the JavaScript logic.

Related Issue(s)

Closes #CALC-3

Description of Changes

• UI: Removed the disabled class from the Multiply (*) button in index.html.
• Logic: Added the case '*' handler to the switch statement in JavaScript to perform the actual multiplication.

Breaking Changes

N/A

Screenshots

Before	After

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module