Removes singleton app lock

[Keboo]

Eliminates the SemaphoreSlim based singleton application lock mechanism. This change simplifies the application's lifecycle by removing unnecessary synchronization during startup and shutdown.

• Removes the acquisition of the singleton lock during application launch.
• Removes the SemaphoreSlim object from the internal application state.
• Removes the release of the singleton lock during application disposal.
• Refactors the internal App class to use primary constructor syntax for improved readability.