Add a Transfer record {name, received, total, incoming, done} and an engine-side
transfers map updated by the RTC/LAN transports — fine-grained on receive (per
chunk in apply_file_frame), per-chunk on send. Engine.transfers() exposes
in-flight transfers (completed linger ~6s, then prune). Both apps poll it and
show a progress row with a bar + percent above the feed.
Verified with examples/progress_demo.rs: a 5MB file's transfer reports
received/total advancing to done.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Patrick Ecord
c9546ac222
