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/fluksio/cli.py

@@ -356,6 +356,14 @@ def _free_port(host: str, start: int) -> int:
 
 
 def cmd_serve(args: argparse.Namespace) -> int:
+    # At a terminal this is a dashboard with the engine as a child of it. The
+    # import is here rather than at the top because it is only ever needed on
+    # that path, and `serve` in a container must not pay for it.
+    if not args.plain and sys.stdout.isatty() and sys.stdin.isatty():
+        from fluksio.tui import run_tui
+
+        return run_tui(args)
+
     data_dir = _data_dir(args.data_dir, args.shared)
     for flag, name in CONCURRENCY_FLAGS.items():
         value = getattr(args, flag, None)
@@ -566,6 +574,11 @@ def _parser() -> argparse.ArgumentParser:
         metavar="N",
         help="python worker processes (default 4, FLOW_MAX_WORKERS)",
     )
+    serve.add_argument(
+        "--plain",
+        action="store_true",
+        help="the log stream rather than the dashboard (the default with no terminal)",
+    )
     serve.add_argument(
         "--gpus",
         type=_at_least(0),