fix(windows): guard titlebar_dark with sapp.isvalid()

[MartenH]

Found while bringing up vlang/gui on native Windows, but it's a real ordering bug independent of toolchain (reproduces under both mingw-w64/gcc and MSVC).

titlebar_dark() calls sapp.win32_get_hwnd(), which asserts _sapp.valid and aborts if sokol_app hasn't been initialised yet:

titlebar_dark() -> win32_get_hwnd() -> abort on _sapp.valid

set_theme() (and any early dark/light toggle) can run before sapp.run(), so on Windows the app aborts at startup before the window even opens.

This guards the DwmSetWindowAttribute call with sapp.isvalid(), so an early call is a harmless no-op; once the window is up the titlebar attribute is applied exactly as before.

pub fn titlebar_dark(dark bool) {
	$if windows {
		if sapp.isvalid() {
			C.DwmSetWindowAttribute(sapp.win32_get_hwnd(), 20, &dark, sizeof(dark))
		}
	}
}

v fmt-clean. Verified on native Windows (mingw-w64 and MSVC builds both start cleanly with the guard).