Rewrite the UI with Avalonia instead of WPF

[TolikPylypchuk]

WPF is great and I feel like I have a good grasp on it, but it's a little dated and it's Windows-only. Avalonia instead is the hot new thing, and cross-platform. So if I use it instead of WPF or, say, WinUI 3, I'll be able to build this app for Linux as well (and maybe even macOS, though it's unlikely). Also, I will switch to Fluent Design instead of Material Design. Additionally, I will be able to implement dynamic themeing/localization more easily if they are considered from the beginning.