TCP thread never reports status, so the throughput indicator can never appear

[Phlabry]

Describe the bug
FromTcpThreadMessage::{Connected, Disconnected, Sent, SendError} are declared (relay/src/message.rs:57-66) and handled (relay/src/update.rs:71-85), but never constructed anywhere. The TCP thread never calls send_to_parent.

Two consequences:

1. on_tcp_packet_sent is only called from the unreachable Sent arm, so show_metrics stays false forever and the throughput block always early-returns (relay/src/view.rs:92-95). The packet rate indicator added in Add a status indicator to the Relay GUI that shows packet transfer rate over TCP #109 cannot ever display.
2. state.tcp_connected is only updated by pressing Check TCP Connection Status (relay/src/update.rs:106-109), so the status line shows stale data until manually polled.

Expected behavior
The TCP thread reports connect, disconnect, and send events, so the throughput indicator works and the status line updates on its own.

Additional context
This makes the acceptance criteria of #109 unmet in the shipped code. The whole metrics subsystem (packets_last_60s, bps, sent_packet_times, human_bps) is currently unreachable.

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