refactor: move platform-specific code to platform_util.py

[Eashan-H]

Purpose

Refactor the Tool Manager subsystem to centralize platform detection and improve cross platform support.

Previously platform specific logic such as OS detection, backend, GUI selection, and OS labeling were duplicated across multiple files.

Approach

• Added platform_utils.py as a centralized platform utility module.
• Removed duplicated platform detection code from gui_fixed.py.
• Added platform aware backend selection in main.py.
• Added dynamic OS labels instead of hardcoded Windows-specific labels.
• Updated uninstall and installation workflows to use the selected backend dynamically.

Benefits

• Reduces code duplication.
• Improves maintainability.
• Centralizes platform specific behavior.
• Provides a cleaner foundation for all OS.
• Makes future platform related changes easier to implement.