Build failure on Windows MSVC: error C7555 due to C++20 designated initializers

When building on Windows using MSVC (eg via GitHub Actions) the build fails during the compilation of main_widget.cpp. The compiler throws an error regarding the use of designated initializers, which are a C++20 feature.

Currently, pdf_viewer_build_config.pro globally sets CONFIG += c++17. MSVC strictly enforces standard flags and refuses to compile C++20 syntax under a C++17 configuration.

Error Log

D:\a\sioyek\sioyek\pdf_viewer\main_widget.cpp(11158,9): error C7555: use of designated initializers requires at least '/std:c++20' [D:\a\sioyek\sioyek\sioyek.vcxproj]
D:\a\sioyek\sioyek\pdf_viewer\main_widget.cpp(11164,9): error C7555: use of designated initializers requires at least '/std:c++20' [D:\a\sioyek\sioyek\sioyek.vcxproj]

To reproduce the behavior run a Windows build using MSVC and Qt 6.11.

Proposed Fix

Update pdf_viewer_build_config.pro to append /std:c++20 to QMAKE_CXXFLAGS inside the win32 block. This satisfies MSVC's requirement for designated initializers without altering the standard for macOS and Linux builds.