Add file transfer methods

Adding methods for transferring files in the BBS style (xmodem etc) besides being useful, improves the "look and feel" of BBS.

Start by reviewing the Go repository structure and existing terminal-sharing entry points, then determine how BBS-style protocols such as XMODEM fit the current design. Define the supported transfer methods and verify transfers end to end; the issue names no files or tests, so the completion criteria need clarification.

Written by the indexing model from the issue text.