ci/ui-test: key concurrency by head sha, not gitea.ref

`gitea.ref` differs between push (`refs/heads/<branch>`) and
pull_request (`refs/pull/N/head`) events even for the same commit,
so the two parallel runs land in different concurrency groups and
the Vite-on-:5173 collision is not suppressed. Switching the key to
the head sha (`gitea.event.pull_request.head.sha || gitea.sha`)
collapses both events into one bucket, leaving exactly one ui-test
alive per commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ui-test.yaml

@@ -19,10 +19,10 @@ on:
 # Playwright launches its own `pnpm dev` on :5173. In host-mode the
 # runner shares the host's port namespace, so two parallel ui-test
 # jobs (e.g. a push event racing with a pull_request event for the
-# same commit) collide on EADDRINUSE. Group by branch/ref and cancel
-# the in-progress run so only one ui-test is alive at a time per ref.
+# same commit) collide on EADDRINUSE. Group by the head commit so
+# push and pull_request events for the same sha share one bucket.
 concurrency:
-  group: ui-test-${{ gitea.ref }}
+  group: ui-test-${{ gitea.event.pull_request.head.sha || gitea.sha }}
   cancel-in-progress: true
 
 jobs: