Upgrade project to 8.0 or later

[unitycoder]

this would allow using better Select Folder dialog (and maybe other speed improvements)
https://devblogs.microsoft.com/dotnet/wpf-file-dialog-improvements-in-dotnet-8/

Previously i've tested upgrading, but many things broke down..

• test upgrade to 8.0 or later (currently uses 4.8) *vs2026 has deprecated Upgrade Assistant, need to use Copilot..
• check that all features work
• can it build single exe builds?
• update github actions
• update installer builds