Add missing GetVirtualScreenSize stub to HeadlessWrapper

Launch.lua calls GetVirtualScreenSize() in DrawPopup() and during
restart, but HeadlessWrapper.lua only stubs GetScreenSize() and
GetScreenScale(). When running headless and an error triggers
DrawPopup on the first OnFrame, this causes a crash because
GetVirtualScreenSize is not yet defined (Modules/Common.lua where
it is normally defined hasn't loaded yet).

The stub delegates to GetScreenSize() since GetScreenScale() returns
1 (no scaling) in headless mode, matching what the real implementation
in Common.lua would produce.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: YangLeoZhao

src/HeadlessWrapper.lua

@@ -51,6 +51,9 @@ end
 function GetScreenScale()
 	return 1
 end
+function GetVirtualScreenSize()
+	return GetScreenSize()
+end
 function GetDPIScaleOverridePercent()
 	return 1
 end