ci: build admin test bundle once and run partitions against it

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -71,13 +71,15 @@ jobs:
         run: cd ui/desktop && npx ember test --test-port 0
 
       - name: test admin ui
-        # Partitions run one after another (one browser at a time): four
-        # concurrent headless browsers stalled on the small shared runners.
+        # The test build is made once and the partitions then run against the
+        # static output, one browser at a time. Rebuilding in-process (and four
+        # concurrent browsers) stalled on the small shared runners.
         run: |
           cd ui/admin
+          npx ember build --environment=test --output-path="$RUNNER_TEMP/admin-test-dist"
           for p in 1 2 3 4; do
             echo "::group::admin partition $p/4"
-            timeout 25m npx ember exam --test-port 0 --split=4 --partition=$p
+            timeout 25m npx ember exam --test-port 0 --path "$RUNNER_TEMP/admin-test-dist" --split=4 --partition=$p
             echo "::endgroup::"
           done