The sender only handled regular files: a staged directory would fail at
upload time, so users could only send individual files.
Now a staged directory is walked recursively and each file is advertised
with a name relative to the folder's parent (e.g. "Trip/day1/img.jpg"),
which is the LocalSend-compatible way to carry structure. The receiver
recreates those subdirectories under the receive dir, creating parents as
needed. The path-traversal guard is preserved: names are cleaned against a
leading "/" to collapse "..", and a containment check ensures the result
stays within the receive dir.
In the TUI send picker, "a" stages the folder currently being browsed;
staged folders are tagged in the panel and the help text is updated.
Tests: directory expansion produces relative names; an end-to-end folder
send recreates the structure on the receiver; destPath preserves subdirs,
rejects traversal, and de-duplicates within subfolders.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
