Try and preserve the wideness of std::filesystem::path more (#660)

As noted in #567, a filesystem path that contains non-ascii will break a
lot if using a clang toolchain.

Luckily, fmtlib has a lossy utf8 convert when you use it to print a path
(after including `fmt/std.h`). The vast majority of this diff is just
removing `.string()` from paths inside loggings calls.

There are some callsites I _didn't_ touch, mainly the options, because
it would be an ABI break to change those to be wide strings and I cbf
looking into settings upgrades. There are also some spots (avs mountpath
remapping, for example) where the path is guaranteed to be ascii, so I
didn't modify them.

ImGui doesn't appear to easily support wide strings (I mean, surely it
does, but I'm not gonna look too far into it) so I mostly just left
those alone too, with a few spots modified to re-use fmtlib's lossy
utf8.

Some of the changes are basically never gonna be hit IRL, like who would
put a file with a non-ascii _extension_ along with their modules? But
the diff is (I hope) pretty easy to validate as OK.

Testing has been somewhat minimal, I fired up the GCC build of spice2x
in a dodgy folder name, got mojibake (running via wine in linux so take
that as you will), ran the unmodified clang spice and crashed the same
way the reporter did. After modification, I get the exact same mojibake
so I assume if the terminal enjoys utf8 it'll display OK.

Claude (only used for review) thinks the commit is fine but is annoyed
that I use `fmt::detail` in the appdata censoring, which is part of the
private API; personally I don't care because it's pretty stable.

src/spice2x/hooks/avshook.cpp

@@ -291,7 +291,7 @@ static int avs_fs_mount(const char *mountpoint, const char *fsroot, const char *
         std::filesystem::create_directories(mapped_path, err);
 
         if (err) {
-            log_warning("hooks::avs", "failed to create '{}': {}", mapped_path.string(), err.message());
+            log_warning("hooks::avs", "failed to create '{}': {}", mapped_path, err.message());
         } else {
 
             // if this is the `e:\`, then create the special directories
@@ -305,7 +305,7 @@ static int avs_fs_mount(const char *mountpoint, const char *fsroot, const char *
 
             log_misc("hooks::avs", "source directory '{}' remapped to '{}'",
                     fsroot,
-                    mapped_path.string());
+                    mapped_path);
         }
 
         new_fs_root = mapped_path.string();