Make Avalonia frontend the default

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• default launcher selection to modern Avalonia unless UseClassicMode is explicitly enabled
• add UseClassicMode setting and wire the existing "Use classic mode" setting UI to it
• prefer current process architecture when resolving development Avalonia builds
• update launcher tests for the new default and dev-output architecture preference

Validation

• dotnet test .\src\UniGetUI.Tests\UniGetUI.Tests.csproj --filter ModernAppLauncherTests --verbosity q --nologo -m:1 /p:BuildInParallel=false
• dotnet build .\src\UniGetUI.Windows.slnx --verbosity q --nologo -m:1 /p:BuildInParallel=false
• launched built x64 WinUI launcher and verified it started UniGetUI.Avalonia.exe