Add standalone Tkinter calculator app with optional headless screenshot

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This PR adds a simple calculator application implemented in Python using Tkinter. The app provides a graphical interface with:

• A display area for current input and results
• Buttons for digits (0–9)
• Buttons for basic operations (+, -, ×, ÷)
• A clear button (C)
• An equals button (=)

Keyboard input is supported for digits and operators, and pressing Enter evaluates the current expression. The × and ÷ symbols in the UI are converted to * and / for evaluation under the hood. Expression evaluation uses Python’s eval with restricted builtins.

For CI and non-interactive environments, the application supports an optional screenshot mode that renders the UI and saves it to calculator_screenshot.png:

# Run interactively
python3 calculator.py

# Generate a screenshot (no persistent window)
python3 calculator.py --screenshot
# or
CALC_SCREENSHOT=1 python3 calculator.py

The README has been updated with basic usage instructions, and a generated screenshot (calculator_screenshot.png) is included in the repository for visual verification.

Fixes #2.

Warning

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