Open a dashboard when serve is run at a terminal

`fluksio serve` printed a log stream and nothing else, so watching an engine
meant a second terminal running `status --watch`, and stopping or pairing it
meant a third. At a terminal it now opens a dashboard: the health and flow
overview `status` draws, the recent runs as a table, and the engine's own
output in a pane below — which is what the earlier decision against this
was protecting, and it is still all there.

The engine is a child process running `serve --plain`, not a thread, so it
outlives the dashboard: q leaves it running and says so, s and r stop and
restart it, c cancels the selected run and e pairs with a portal. An engine
already serving this directory is adopted rather than duplicated, and it can
be stopped from here only because the pidfile and the token together prove
it is this installation's.

`--plain` and no terminal both keep the old behaviour, which is what the
container and CI run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/tests/test_cli.py

@@ -567,11 +567,51 @@ def test_run_and_sweep_take_what_to_sync() -> None:
     assert parser.parse_args(["sweep", "train"]).sync == []
 
 
+def test_the_dashboard_runs_the_engine_as_a_child_of_itself(monkeypatch) -> None:
+    """At a terminal `serve` is a dashboard; the engine is a plain serve.
+
+    Every flag is passed through, so what the child runs with is what serve
+    was asked for — and `--plain` is what stops it opening a second one.
+    """
+    import sys
+
+    from fluksio import cli
+    from fluksio.tui import child_argv
+
+    argv = child_argv(["serve", "--port", "8123", "--gpus", "1"])
+    assert argv[:3] == [sys.executable, "-m", "fluksio.cli"]
+    assert argv[3:] == ["serve", "--port", "8123", "--gpus", "1", "--plain"]
+    # Already plain: told once, not twice.
+    assert child_argv(["serve", "--plain"])[3:] == ["serve", "--plain"]
+
+    opened: list[str] = []
+    monkeypatch.setattr(
+        "fluksio.tui.run_tui", lambda args: opened.append("tui") or 0, raising=False
+    )
+    monkeypatch.setattr(sys.stdout, "isatty", lambda: True, raising=False)
+    monkeypatch.setattr(sys.stdin, "isatty", lambda: True, raising=False)
+
+    parser = cli._parser()
+    assert cli.cmd_serve(parser.parse_args(["serve"])) == 0
+    assert opened == ["tui"]
+
+    # `--plain` goes past it, which is what the child and every container does.
+    # Nothing else of serve runs here, so it fails on the data directory it is
+    # given rather than opening a dashboard.
+    opened.clear()
+    monkeypatch.setattr(
+        cli, "_data_dir", lambda *a, **k: (_ for _ in ()).throw(SystemExit(3))
+    )
+    with __import__("pytest").raises(SystemExit):
+        cli.cmd_serve(parser.parse_args(["serve", "--plain"]))
+    assert opened == []
+
+
 def test_a_serving_engine_records_itself_until_it_stops(tmp_path) -> None:
     """A pid nobody is running is the same as no pidfile at all."""
     import os
 
-    from fluksio.cli import PIDFILE, read_pidfile, write_pidfile
+    from fluksio.cli import read_pidfile, write_pidfile
 
     assert read_pidfile(tmp_path) is None
 
@@ -586,7 +626,7 @@ def test_a_serving_engine_records_itself_until_it_stops(tmp_path) -> None:
     assert read_pidfile(tmp_path) is None
 
 
-def test_who_holds_the_port_is_told_apart_by_the_token(tmp_path) -> None:
+def test_who_holds_the_port_is_told_apart_by_the_token() -> None:
     """Only this directory's own engine may be reported as already up.
 
     The token is signed with this directory's secret key, so an engine that