Adds LocalSend-compatible text messaging alongside file transfer.
Wire format (matches the LocalSend app): a message is a single "file" with
fileType "text/plain" whose content rides in the prepare-upload `preview`
field. The receiver returns an empty token set, so nothing is uploaded — the
text is read straight from the preview.
- client: SendMessage builds that single-file prepare-upload (no body upload).
- server: detect a message (one text file with non-empty preview), surface it
on a new Messages() channel instead of saving a file, and respond with an
empty file set. Messages bypass the accept prompt (auto-received); the PIN
gate still applies.
- app: bridge the server's messages channel to the TUI as MessageMsg.
- tui: a new Messages tab lists received messages (enter to read full, d to
delete); press `m` on a device to compose and send one. Incoming messages
show a footer notice.
Tests: end-to-end send→receive (text intact, sender preserved, nothing written
to disk) and unit coverage of the message-detection rule.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
