Send Packet button does not send a packet

Describe the bug
The Send Packet handler stores a timestamp and returns (relay/src/update.rs:183-188). It performs no I/O and queues no message. The last_send_timestamp it writes is never read by relay/src/view.rs, so clicking the button produces no packet and no visible change.

To Reproduce

1. Start the relay and connect TCP with a listener running.
2. Press Send Packet.
3. No packet arrives at the listener and the GUI does not change.

Expected behavior
Clicking sends one packet, which was acceptance criterion 3 of #107.

Additional context
The button only renders as active when baton is connected (relay/src/view.rs:237-243), which makes the no-op more misleading. Also note .unwrap() on duration_since(UNIX_EPOCH) at relay/src/update.rs:185 is a latent panic.

Found while writing the setup documentation in #138 / #178.