fix(sftp): list the drive root when browsing to "C:" on Windows

[kipavy]

Closes #314.

Cause

On Windows a bare drive spec is drive-relative: C: means "the current directory on drive C", not the drive root. fs_list_dir passed the path straight to std::fs::read_dir, so browsing to C: listed the process CWD on that drive — the install directory (uninstall.exe, voltius.exe). C:\ worked, which matches the report.

Three places in the Local Machine pane produced a bare C:: the up-button (computeParentDir returned parts[0] with no separator), the C: breadcrumb crumb, and typing C: in the path bar.

Fix

• fs_list_dir normalizes a bare drive spec to C:\ before reading it, which covers all three entry points regardless of what the frontend sends.
• parentDir returns C:\ rather than C: for a drive root, handles UNC roots, and returns "" when there is no parent.
• The breadcrumb emits C:/ for the drive crumb, and new files/folders are joined with joinPath instead of a hardcoded /.

Also folds FilePane's own parent-directory copy and its three "parent dir + join name" copies (rename, compress, extract) into the shared parentDir/joinPath helpers.

Tests

• New Rust unit test for the drive-root normalization; cargo test --lib commands::fs passes.
• New parentDir/joinPath cases for drive roots, UNC roots and roots without a parent; vitest run src/components/filetransfer (21 files, 101 tests) and tsc --noEmit pass.

Not reproducible on Linux, so the Windows behaviour itself still wants a manual check on the reporter's setup.