ci: run Playwright test suite on PRs and main

Add a Tests workflow that installs deps, installs the Chromium browser, and
runs `yarn test` on every pull_request and on push to main. Previously the
only CI workflows built the preview/release bundles and never executed the
test suite, so regressions could merge unverified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q1PucHHFUDRRo2g7zC6aLq

Author: claude

.github/workflows/test.yml

@@ -0,0 +1,29 @@
+name: Tests
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  playwright:
+    name: Playwright tests
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '18'
+          cache: 'yarn'
+
+      - name: Install dependencies
+        run: yarn install --frozen-lockfile
+
+      - name: Install Playwright browsers
+        run: npx playwright install --with-deps chromium
+
+      - name: Run Playwright tests
+        run: yarn test