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/integration.yaml

@@ -37,6 +37,14 @@ on:
       - '.gitea/workflows/integration.yaml'
       - '!**/*.md'
 
+# Shared concurrency group with go-unit and ui-test: the host runner uses
+# one workspace, so these jobs must not run concurrently or they race on
+# workspace teardown (see go-unit.yaml for the full rationale). Runs are
+# queued, not cancelled.
+concurrency:
+  group: galaxy-host-runner
+  cancel-in-progress: false
+
 jobs:
   integration:
     runs-on: ubuntu-latest