First of all, thanks for this extremely useful project!
I would like to have it run in the browser by compiling it to web-assembly using emscripten. Emscripten itself does not currently provide support for std::filesystem and it seems to me this might be in scope for your library.

I was able to have it compile using emscripten by simply adding

#elif defined(__EMSCRIPTEN__)
#define GHC_OS_WEB

to your platform determination part in filesystem.hpp.

It was immediately usable, but the following simple example gives a crash, so I suppose there may be some more effort necessary to get things running correctly:

for(auto& p: fs::recursive_directory_iterator("."))
        std::cout << p.path() << '\n';

fails after two prints

./proc
./proc/self
Operation not permitted: './proc/self'

Note that the "filesystem" in this case is the virtual browser filesystem -- I have not yet researched the nuances that might make the browser platform different from others.