feat: add pet size customization and fix tray skin selection

[mx57]

• Added pet_size configuration option (range 50–250px) to ConfigManager with default 100.
• Implemented size_slider with real-time value indicator in SettingsDialog.
• Added set_pet_size method to PetWindow to dynamically adjust window size and animation renderer.
• Scaled petting interaction radius dynamically based on pet_size in InputManager.
• Fixed TrayMenu.select_skin to ensure skin selection from system tray menu is saved to ConfigManager.
• Added unit tests for pet size configuration, dynamic window resizing, and tray menu skin persistence.

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PR created automatically by Jules for task 15474643929253444452 started by @mx57

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