Update, morning after Jam deadline: I switched back to an SDL3 backend, and that seems to have fixed

• setting the icon for the window & taskbar
• copying characters correctly

The downside is that the window is resizable again, which I don't want, but is less important than the above two issues. (This is because I'm having a hard time calling Win32 functions with the hWnd that the SDL backend gives me.) DVUI's different backends are buggy in a few ways, I've come to find out.