Add delivery confirmation to peer-to-peer file transfer

## Problem

`uploadFiles()` marks a sent file "Sending" once when the transfer starts and never updates that status afterward, even once the whole chunked transfer over the data channel has actually completed. There's currently no way to tell "sent," "still sending," and "peer never received it" apart in the Files panel UI.

## Proposed solution

- Have the receiving side send an explicit acknowledgement event back over the data channel once a file's chunks are fully reassembled.
- Update the sender's file-list state (`"sending" → "delivered"`) when that ack arrives, and surface a distinct visual state in the Files panel for each of "sending," "delivered," and (after some timeout with no ack) "undelivered."
- Verify with two independent real browser sessions, the same technique already used to verify the rest of the realtime feature set — see the two-participant testing approach described in [`docs/testing.md`](https://github.com/hoangsonww/Threadline-RealTime-Collab/blob/main/docs/testing.md).

## References

- [`docs/roadmap.md`](https://github.com/hoangsonww/Threadline-RealTime-Collab/blob/main/docs/roadmap.md) — "No delivery confirmation"
- [`docs/realtime.md#screen-sharing`](https://github.com/hoangsonww/Threadline-RealTime-Collab/blob/main/docs/realtime.md)