Install yalc globally in Playwright CI workflow

The dummy app's preinstall script requires yalc to be available globally
to link the local react-on-rails package during yarn install.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: justin808

.github/workflows/playwright.yml

@@ -22,6 +22,9 @@ jobs:
           node-version: '20'
           cache: 'yarn'
 
+      - name: Install yalc globally
+        run: npm install -g yalc
+
       - name: Install root dependencies
         run: yarn install