Fix script injection in PR Playground preview workflow

.github/workflows/pr-playground-preview.yml

@@ -15,6 +15,9 @@ permissions:
 jobs:
     preview:
         runs-on: ubuntu-latest
+        # Fork PRs get a read-only token, so the branch push below would fail.
+        # Skip on forks rather than letting the push step hard-fail.
+        if: github.event.pull_request.head.repo.full_name == github.repository
         steps:
             - name: Checkout
               uses: actions/checkout@v4
@@ -29,8 +32,14 @@ jobs:
               run: npm i
 
             - name: Update version placeholder
+              env:
+                  PR_NUMBER: ${{ github.event.pull_request.number }}
               run: |
-                  sed -i "s/__VERSION__/Pull Request #${{ github.event.pull_request.number }} - ${{ github.event.pull_request.title }}/g" popup.php
+                  # PR_NUMBER is numeric, so it is safe to substitute. Never
+                  # interpolate the PR title here — it is attacker controlled and
+                  # would run as shell on the runner.
+                  sed -i "s/__VERSION__/0.0.0-pr${PR_NUMBER}/g" popup.php
+                  grep -n "Version:" popup.php
 
             - name: Build
               run: npm run build