Flatten Windows publish layout and fix restart handoff

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• flatten the Windows publish output so UniGetUI.exe, UniGetUI.Avalonia.exe, and pinget.exe ship side by side
• publish the bundled Avalonia app and pinget once, then merge them into the final Windows publish output
• extract a shared pre-UI startup dispatcher and split Avalonia startup into a dedicated host path
• fix classic/modern restart handoff by relaunching only after the current process exits
• make Avalonia resolve UniGetUI Elevator.exe correctly in the flat side-by-side layout, with fallback for the older subfolder layout

Validation

• dotnet build UniGetUI.Avalonia/UniGetUI.Avalonia.csproj -c Debug -p:Platform=x64 -r win-x64