Add Stack-Based GUI Calculator with Powers, Roots, and Reciprocations

What it does:

• This PR adds a GUI-based calculator using TKinter with the following features:
• Stack-based evaluation using infix to postfix conversion (no eval() used).
• Standard arithmetic operations: +, -, *, /, %, ^.
• Supports square root via a^(1/2) and reciprocal (1/x) via a^(-1).
• Handles negative numbers and parentheses correctly.
• Simple, clickable TKinter buttons for all operations, numbers, and clear (C).

Example Usage:

• 5^(-1) evaluates 1/5 = 0.20
• 16^(1/2) evaluates sqrt(16) = 4.0
• (-3+2)^2 evaluates to 1