Run Playwright on all platforms in PRs (#7304)

# Description of Changes
Nightlies keep failing because the Playwright tests only run on Chrome
in PRs. This PR changes it so that we run all 3 browsers in all
(frontend) PRs so we catch these things before they merge in. They run
in parallel so it won't take any more time for the CI to finish.

.github/workflows/e2e-stubbed.yml

@@ -2,7 +2,8 @@ name: Playwright E2E (stubbed)
 
 # Reusable workflow called from build.yml. Backend-free Playwright suite —
 # fast, no Spring Boot required. Runs against the `stubbed` project which
-# mocks API responses in the browser.
+# mocks API responses in the browser. Fans out one job per browser
+# (chromium/firefox/webkit) so all three run in parallel on their own runner.
 on:
   workflow_call:
 
@@ -11,7 +12,19 @@ permissions:
 
 jobs:
   playwright-e2e:
+    name: playwright-e2e (${{ matrix.browser }})
     runs-on: ubuntu-latest
+    strategy:
+      # One browser breaking must not mask a failure in another - report all.
+      fail-fast: false
+      matrix:
+        include:
+          - browser: chromium
+            project: stubbed
+          - browser: firefox
+            project: stubbed-firefox
+          - browser: webkit
+            project: stubbed-webkit
     steps:
       - name: Harden Runner
         uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
@@ -27,16 +40,16 @@ jobs:
           cache-dependency-path: frontend/package-lock.json
       - name: Install Task
         uses: go-task/setup-task@01a4adf9db2d14c1de7a560f09170b6e0df736aa # v2.1.0
-      - name: Install Playwright (chromium only)
-        run: task e2e:install -- chromium
+      - name: Install Playwright (${{ matrix.browser }})
+        run: task e2e:install -- ${{ matrix.browser }}
       - name: Build frontend (production bundle for vite preview)
         env:
           VITE_BUILD_FOR_PREVIEW: "1"
         run: task frontend:build
-      - name: Run stubbed E2E tests (chromium)
+      - name: Run stubbed E2E tests (${{ matrix.browser }})
         env:
           PLAYWRIGHT_JSON_OUTPUT_FILE: ${{ github.workspace }}/frontend/playwright-report/results.json
-        run: task e2e:stubbed -- --workers=3
+        run: task e2e:stubbed-project PROJECT=${{ matrix.project }} -- --workers=3
       - name: Flag flaky tests
         # Runs regardless of the test outcome: a flaky test (passed on retry)
         # leaves the step green, so this is the only place it surfaces. Emits
@@ -50,6 +63,6 @@ jobs:
         if: always()
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: playwright-report-stubbed-${{ github.run_id }}
+          name: playwright-report-stubbed-${{ matrix.browser }}-${{ github.run_id }}
           path: frontend/playwright-report/
           retention-days: 7