Add “Start with Windows” via StartupTask (MSIX/Store only)

[Marko19907]

Implement auto-start on login for the Microsoft Store / MSIX packaged build using Windows StartupTask (manifest-based startup). This will not be available for the portable build which is mostly because of these two reasons:

1. StartupTask is the most “Windows-native” approach for packaged apps, it shows an entry in Startup Apps UI / Task Manager
2. Keeps the portable version truly portable: no registry writes, no leftover startup entries if the user deletes the exe

An in-app toggle for the MSIX build could be added, for example, a menu item that calls the StartupTask enable/disable APIs. Manifest-only is sufficient for now I think