Fix race condition on startup and improve fault tolerance

[Gabriel Dufresne (GabrielDuf)]

This pull request improves the robustness of package manager initialization and error reporting throughout the application, ensuring that failures in constructing individual package managers do not crash the entire app. It also enhances exception logging and updates code to handle package managers that may be unavailable at runtime.

Package Manager Initialization and Handling

• Refactored PEInterface so that each package manager (WinGet, Scoop, Chocolatey, etc.) is now initialized via a safe Create method that catches and logs exceptions, returning null if construction fails. This prevents a single manager's failure from crashing the app during static initialization. (src/UniGetUI.PackageEngine.PackageEngine/PEInterface.cs src/UniGetUI.PackageEngine.PackageEngine/PEInterface.csL36-R114)
• Updated all usages of package manager fields to handle the possibility of null values, including in IgnoredUpdates.xaml.cs, MainView.xaml.cs, and PackageManager.xaml.cs, preventing null reference exceptions and gracefully skipping unavailable managers. (src/UniGetUI/Pages/DialogPages/IgnoredUpdates.xaml.cs src/UniGetUI/Pages/MainView.xaml.cs src/UniGetUI/Pages/SettingsPages/ManagersPages/PackageManager.xaml.cs

Exception Logging Improvements

• Enhanced the crash handler to include the full exception detail using Exception.ToString(), ensuring that all inner exceptions (including those in AggregateException) are logged, not just the direct InnerException chain. (src/UniGetUI/CrashHandler.cs

Application Startup Stability

• Ensured that Avalonia's UI-thread dispatcher is bound to the main thread before starting the single-instance listener, preventing threading issues if a second instance connects during startup. (src/UniGetUI.Avalonia/Infrastructure/AvaloniaAppHost.cs