✨ Complete: Customizable Pet Size Implementation

[mx57]

This change completes the customizable pet size feature across the application.

• Added default pet_size: 100 to ConfigManager.
• Added pet size slider (50px to 250px) with real-time value label indicator to SettingsDialog.
• Added set_pet_size method to PetWindow for live resizing and connected it in TrayMenu.
• Updated InputManager to calculate petting interaction radius dynamically based on pet window size instead of hardcoded 60px.
• Added test cases in tests/test_utils.py verifying config defaults, window resizing, and dynamic interaction radius scaling.

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PR created automatically by Jules for task 16539117085559937780 started by @mx57

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