[common/fs] Preserve Windows roots during path sanitization (#4437)

Commit 5b4c29b123 made SanitizePath rebuild paths from split components after resolving traversal. That reconstruction retained only one leading separator, collapsing UNC paths and breaking network-backed directories.

Parse the path root separately from normal components. Preserve UNC prefixes, drive-absolute and drive-relative forms, and extended UNC/device namespaces while normalizing separators and dot components.

Treat the UNC server/share portion as part of the root so parent traversal cannot escape into another share. Also preserve root-only results such as / and C:\\ instead of stripping their final separator.

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-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4437
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/common/fs/path_util.h

@@ -347,8 +347,9 @@ enum class DirectorySeparator {
 // i.e. "C:\Users\Yuzu\Documents\save.bin" becomes {"C:", "Users", "Yuzu", "Documents", "save.bin" }
 [[nodiscard]] std::vector<std::string> SplitPathComponentsCopy(std::string_view filename);
 
-// Removes trailing slash, makes all '\\' into '/', and removes duplicate '/'. Makes '/' into '\\'
-// depending if directory_separator is BackwardSlash or PlatformDefault and running on windows
+// Normalizes directory separators, removes duplicate and non-root trailing separators, and resolves
+// '.' and '..' components without traversing above the path root. Windows drive and UNC roots are
+// preserved.
 [[nodiscard]] std::string SanitizePath(
     std::string_view path,
     DirectorySeparator directory_separator = DirectorySeparator::ForwardSlash);