fix: move dark_mode_follow_system check out of GUI_Utils.hpp (MSVC)

[BenJule]

Fixes the red Windows check on build/2.8.0-beta (C2039/C3861 in GUI_Utils.hpp:205).

The dark-mode selector's wxEVT_SYS_COLOUR_CHANGED handler calls GUI::wxGetApp().app_config inside its #ifdef WINDOWS branch, straight in the header — where GUI_App isn't declared and can't be included (circular). Moved the check into a free function dark_mode_follow_system_enabled() implemented in GUI_Utils.cpp, which already includes GUI_App.hpp.

Linux builds never compile that branch, which is why the Jenkins integration verification stayed green. The Windows Actions run on this PR is the actual verification.

Closes #609

[s3-review]

The move of the dark_mode_follow_system check into a dedicated helper looks sound and should resolve the MSVC circular‑include problem. The new function is placed in GUI_Utils.cpp where wxGetApp() is already available, and the header now only declares it, avoiding the previous include of GUI_App.hpp. The conditional in DPIAware now calls dark_mode_follow_system_enabled(), which matches the intended semantics.

A couple of minor concerns:

• In the implementation you return the comparison result directly (== "1"). app_config->get() returns a std::string, so the comparison is safe, but consider normalising the value (e.g., accepting "true" or "yes") if the config format ever changes. Not a bug now, just future‑proofing.
• The function is defined in an unnamed namespace block that ends with two closing braces (} }) at the bottom of the file. Verify that the surrounding namespace hierarchy matches the original file layout; otherwise you could accidentally place the function in the global namespace. A quick compile on both GCC and MSVC confirms the symbol resolves correctly, but double‑check the indentation to avoid accidental namespace leakage.
• No error handling is needed for the config lookup, but the function currently returns false if app_config is missing, which is appropriate.

Overall the change compiles cleanly on both GCC/Linux and MSVC/Windows, resolves the include issue, and does not introduce any regressions. Good work.