ci/ui-test: serialise per-ref + clear stale Vite before Playwright

Two ui-test jobs cannot coexist on the same host: Playwright's
`webServer` spec spawns `pnpm dev` on :5173, and on a host-mode
runner the port lives in the host namespace shared by every job.
ui-test #67 hit "Error: http://localhost:5173 is already used"
because a parallel job's Vite still held the port.

Two changes:

1. `concurrency: ui-test-${{ gitea.ref }}` with `cancel-in-progress:
   true`. New push/PR runs against the same ref kill any earlier
   ui-test before starting, so we never have two `pnpm dev`s alive
   at once.
2. `pkill -f 'vite dev' || true` plus `fuser -k 5173/tcp` right
   before Playwright. Defence in depth in case the concurrency
   cancellation does not reap the spawned shell promptly.

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

.gitea/workflows/ui-test.yaml

@@ -16,6 +16,15 @@ on:
       - '.gitea/workflows/ui-test.yaml'
       - '!**/*.md'
 
+# 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.
+concurrency:
+  group: ui-test-${{ gitea.ref }}
+  cancel-in-progress: true
+
 jobs:
   test:
     runs-on: ubuntu-latest
@@ -59,6 +68,15 @@ jobs:
         working-directory: ui/frontend
         run: pnpm test
 
+      - name: Clear stale Vite from :5173
+        # Defence in depth in case a previous job's webServer survived
+        # the concurrency-cancel — `pkill` does not fail when there is
+        # nothing to kill, and `fuser -k` cleans up anything else
+        # holding the port.
+        run: |
+          pkill -f 'vite dev' || true
+          fuser -k 5173/tcp 2>/dev/null || true
+
       - name: Run Playwright
         working-directory: ui/frontend
         run: pnpm exec playwright test