The smoke test parsed the listening port out of the dev server's stdout,
but the server announces it from the main thread while the logger thread
writes to the same stream. A log line can land inside the announcement —
Arch CI caught it — leaving the digits on the next line, so the test read
nothing and failed.
Add --port-file, written through QSaveFile once the server listens, and
read the port from there instead of from the log. The file is either
absent or complete, so logging cannot corrupt the handshake.
