UI: Add application icon for Windows and macOS

[btmlnsn]

Summary
This PR introduces a custom application icon for LabGym containing the LabGym logo, which can now we displayed in the macOS dock and Windows taskbar. This feature improves the user experience by giving the application a more polished and recognizable appearance, likely making it easier for users to identify.

Implementation
To achieve this, a new dedicated helper file "gui_app_icon.py" was created to handle platform-specific logic for setting the application. This new module is responsible for:

• Detecting the operating system (Windows vs macOS)
• Loading the appropriate icon file (.ico for Windows, .icns for macOS).
• Handling platform-specific APIs to set the icon in the dock/taskbar and window frame.
  This approach keeps the icon-related code neatly encapsulated and separate from the main GUI logic.

Benefits

• Improved User Experience: Provides a polished and consistent look and feel.
• Better recognition: Makes LabGym application more easily-identifiable.
• Modular Code: Centralizes the platform-specific logic into a single, maintainable file.

Notes

• A new file, gui_app_icon.py has been added to the LabGym project.
• A new folder, LabGym/assets/icons has been added to the project, containing: labgym.png (the base app icon), labgym.ico (the large app icon for Windows systems), labgym_small.ico (the small app icon for Windows systems, with a simplified logo), and labgym.icns (the LabGym iconset for macOS systems).
• This implementation currently supports Windows and macOS only. Support for displaying the application icon on Linux desktop environments may be added in a future update.

[btmlnsn]

Will correct these CI test errors momentarily - fixes to come with forthcoming commits.

[btmlnsn]

everything seems to be working now - sorry about the failed builds. I've attached a couple quick screenshots of what is being used as an app icon. The more elaborate logo tends to render well on macOS consistently, with sufficient detail. However, Windows-rendered app icons tend to be lower resolution, and the traditional LabGym logo renders poorly on smaller renders due to its detail. Thus, I introduced the alternate logo - the single mouse with the LabGym colors. The more detailed logo is still available for Windows renders, but if Windows renders below a particular size threshold (which tends to be the case), the simplified logo is offered up. Just thought I'd mention it.

By the way, the simplified logo was something made rather quickly for the original purpose of being the tabbed favicon image for the 'under construction' labgym's web landing page - for all the same reasons the image was introduced here.