Fix dpi_aware.cpp guard: OS_WIN -> _WIN32

OS_WIN is a Chromium internal define that is not set in our CMake
compile flags. _WIN32 is always set by MSVC on Windows, so use that
instead. The previous OS_WIN guard caused the entire file to compile
to nothing, leaving IsProcessDpiAware, GetSystemDpi,
GetDpiAwareWindowSize and SetBrowserDpiSettings undefined.

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

Author: linesight

src/client_handler/dpi_aware.cpp

@@ -4,7 +4,7 @@
 
 // Windows only
 
-#ifdef OS_WIN
+#ifdef _WIN32
 
 #pragma comment(lib, "Gdi32.lib")
 #include <math.h>
@@ -229,4 +229,4 @@ void SetBrowserDpiSettings(CefRefPtr<CefBrowser> cefBrowser,
     );
 }
 
-#endif  // OS_WIN
+#endif  // _WIN32