Add cute pastel-themed Tkinter calculator

[amna-2]

Motivation

• Provide a small, user-friendly desktop calculator implemented in Python with a playful pastel UI.
• Ship a self-contained example that demonstrates basic arithmetic, decimals, and parentheses support with simple input controls.

Description

• Add calculator.py which defines CuteCalculator, a Tkinter app with a styled display, color-coded buttons, and a floating ) button for parentheses.
• Implement button handling for digits, operators, C (clear), ⌫ (backspace), and = (evaluate) and wire the app to run with python calculator.py.
• Add a lightweight expression sanitizer that allows only characters in 0123456789+-*/%.() and evaluate expressions with eval while disabling __builtins__ for basic safety.
• Update README.md with run instructions and a short feature list for the new calculator.

Testing

• Ran python -m py_compile calculator.py which succeeded, confirming the file is syntactically valid.
• No automated GUI tests were run; the change contains only the application and docs updates (manual UI verification is expected when launching python calculator.py).

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Make display read-only or sync typed text before evaluation

The Entry is editable, but = evaluates self.expression (built only via button clicks), so any text the user types or pastes into the display is ignored and can leave the shown expression out of sync with what gets computed. In practice, typing 2+2 and pressing = does nothing (or evaluates an older button-built expression), which is a user-visible correctness bug for interactive input.

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