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/pyproject.toml

@@ -50,6 +50,9 @@ dependencies = [
     # `fluksio status` draws with it. Already here underneath fastapi's CLI,
     # named because a command that depends on it should say so.
     "rich>=13",
+    # `fluksio serve` opens a dashboard with it at a terminal. Pure python and
+    # mostly rich underneath, which is already here.
+    "textual>=1.0",
 ]
 
 [project.optional-dependencies]
@@ -152,6 +155,8 @@ ignore = [
 # It talks to whoever ran it; that is what a command line is.
 "fluksio/cli.py" = ["T201"]
 "fluksio/sdk/cli.py" = ["T201"]
+# What it prints is the line left in the terminal after the dashboard closes.
+"fluksio/tui.py" = ["T201"]
 # Node functions take `params` whether or not they use it — that is the
 # contract the engine calls them with.
 "fluksio/flow/nodes.py" = ["ARG001", "ARG002"]