Fix copy name_without_ext trimming folder names with dots

[JustForFun88]

Fixes #3335

[sxyazi]

Thank you for the patch, but this is not the right place to fix the issue - stem() is just a wrapper around Rust's file_stem(), and it should be behave the exact same way, also calling is_dir() on a &Path will involve a IO operation, but stem() should not do any IO (or even allocations). Instead, fix the issue at the upper-level copy command

[JustForFun88]

Thank you for the patch, but this is not the right place to fix the issue - stem() is just a wrapper around Rust's file_stem(), and it should be behave the exact same way, also calling is_dir() on a &Path will involve a IO operation, but stem() should not do any IO (or even allocations). Instead, fix the issue at the upper-level copy command

Done

[sxyazi]

Don't use as_local() since that only supports local files, whereas copy command should be cross-filesystem and support non-local filesystems.

Also, don't do any sync I/O on the main thread, since it can be slow, especially when the user selects many files or the files reside on a remote filesystem mount. Instead, detach those operations to an async blocking thread.

Currently the file selection state only stores the file URL, maybe we could also store a flag indicating whether the file is a directory, which would introduce new cache-invalidation problems though. Maybe the current implementation of real-time I/O isn't a big issue as long as the user doesn't do something crazy...