Fix IntelliSense/MSVC errors and guard wxApp exception overrides

[PeramatoG]

Motivation

• Resolve Visual Studio/IntelliSense parse errors where std::optional appears missing and cascading syntax errors caused by MSVC using a lower language mode.
• Avoid invalid override diagnostics for wxApp exception hooks on builds where wxWidgets disables exception support.

Description

• Add MSVC-specific compile option /std:c++20 via target_compile_options in CMakeLists.txt to enforce C++20 language mode for Visual Studio and align IntelliSense with cxx_std_20.
• Wrap MyApp::OnExceptionInMainLoop and MyApp::OnUnhandledException declarations and definitions in main.cpp with #if wxUSE_EXCEPTIONS so the override specifier is only present when wx exception hooks are enabled.
• Modified files: CMakeLists.txt and main.cpp.

Testing

• Ran tests/check_perastage_tree_modules.sh and it passed (OK).
• Ran tests/check_no_configmanager_get_in_gui.sh and it passed (OK).
• Attempted cmake -S . -B build && cmake --build build -j4 but a full local build could not complete in this environment due to missing wxWidgets development packages, so full compile/link validation was not performed.

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Codex Task