Add % and log() functionality to the calculator

[crigrii]

What was changed

• Replaced the two empty placeholder buttons in the UI with fully functional % and log buttons.
• Modified the input handler so that pressing the log button correctly inserts log( into the expression.
• Added support for logarithmic calculations by exposing math.log inside the evaluator.
• Preserved the original layout, structure, and coding style of the project without introducing unrelated modifications.

Why this change was made

The project originally included two unused buttons intended for future implementation.
Issue #6 specifically requested adding modulo and logarithm support.

This PR resolves that request by:

• Completing those two missing functionalities,
• Expanding the calculator’s mathematical capabilities,
• Maintaining compatibility with the existing architecture,
• Avoiding dead buttons or incomplete UI elements.