ci: serialize go-unit/ui-test/integration on a shared concurrency group

These three jobs run on the host runner, which shares a single workspace.
Run concurrently, they race on workspace teardown: ui-test's production
build leaves a root-owned ui/frontend/build that a sibling job's (non-root)
cleanup cannot remove, which then spuriously fails that job — observed
repeatedly on go-unit when it overlapped a ui-test build. Put all three in
one global concurrency group (galaxy-host-runner, cancel-in-progress:
false) so they run one at a time and never collide. CI becomes more serial
but the false failures stop.

This also subsumes ui-test's former ui-test-singleton group, which guarded
the Playwright :5173 port; the shared group preserves that guarantee.

.gitea/workflows/go-unit.yaml

@@ -30,6 +30,17 @@ on:
       - '.gitea/workflows/go-unit.yaml'
       - '!**/*.md'
 
+# Serialised with ui-test and integration via a shared concurrency group.
+# All three run on the host runner, which shares a single workspace, so
+# running them at the same time races on workspace teardown: the
+# root-owned ui/frontend/build that ui-test's production build leaves
+# behind cannot be removed by a sibling job's (non-root) cleanup, which
+# then spuriously fails that job. One global group runs them one at a
+# time; cancel-in-progress: false queues runs instead of cancelling them.
+concurrency:
+  group: galaxy-host-runner
+  cancel-in-progress: false
+
 jobs:
   test:
     runs-on: ubuntu-latest