Fix LAN dev-connection server leaking port 9000 on stop/reconnect

[bekriebel]

The LAN dev-connection server (port 9000) was never released when dev connection was toggled off or the watch disconnected, and could not rebind on reconnect -- requiring an app force-stop to recover.

startSuspend() returns immediately in this Ktor/CIO engine config, so the restart loop spawned a duplicate server that failed to bind while the real listener leaked unreferenced. stop() then operated on the wrong instance, so the socket was never freed. Additionally, the transport was assigned to last only after start() returns, but start() now suspends indefinitely, so the stop() in teardown always saw null.

Start the server once and await cancellation; record the transport before start(); and explicitly stop() in a finally to free the port on toggle-off and disconnect.

Fixes #219

Co-Authored-By: glm-5.2

Reproduction steps for the issue:

1. Enable Settings -> Phone -> Connectivity -> Use LAN developer connection
2. On a device, enable Dev Connection
3. Using pebble tools, run pebble ping --phone <IP> (works)
4. Turn off Dev Connection
5. Run the ping command again and note that it still works, even though the connection should be off
6. Disconnect the phone either with the Disconnect button, or by toggling Bluetooth on the watch
7. Reconnect the watch
8. Turn on Dev Connection
9. Run the ping command again (fails)