`fs::recursive_directory_iterator` Oof. You might want to look into how voidtools' Everything works before calling this "efficient." (The fastest windows search tools don't actually recurse the filesystem, but instead read the NTFS Master File Table (MFT) directly.)
Also, a bit of advice: separate your business logic (file searching) from your UI logic (rendering the GUI). That'll make refactoring (and unit testing) much easier and helps to avoid tangled spaghetti code as you start building larger applications.