Fix non-Windows builds: expose dsky_backend_t type to stub section

- Add dsky_backend.h include before Windows ifdef block
- Resolves unknown type name error in non-Windows stub implementation
- Enables Linux/macOS compilation of dsky_gui.c
- Maintains Windows functionality while providing cross-platform support

Author: jqntn

Comanche055-C89/dsky_gui.c

@@ -5,6 +5,8 @@
  * Renders the DSKY as a graphical window using native Win32 GDI.
  */
 
+#include "dsky_backend.h"
+
 #ifdef _WIN32
 
 #include <windows.h>