Add workflow to periodically update reference screenshots

Add a scheduled GitHub Actions workflow that runs weekly to update
reference screenshots. The workflow pulls the pre-built testbed image,
sets up the docker environment, runs make docker-update-screenshots,
and creates a pull request if changes are detected.

This helps ensure screenshots stay current and reduces manual maintenance
overhead.

Related to #1068

Author: jelmer

.github/workflows/update-screenshots.yaml

@@ -0,0 +1,67 @@
+name: Update screenshots periodically
+
+on:
+  workflow_dispatch:
+  schedule:
+    # Run weekly, at 3:30pm on Monday
+    # https://cron.help/#30_15_*_*_1
+    - cron: '30 15 * * 1'
+
+env:
+  TESTBED_IMAGE: "ghcr.io/isso-comments/isso-js-testbed:latest"
+
+jobs:
+  update-screenshots:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Switch testbed image to ${{ env.TESTBED_IMAGE }}
+        run: |
+          sed -i "s|isso-js-testbed|${{ env.TESTBED_IMAGE }}|g" docker-compose.yml
+          # Also remove the `build`: section so image is pulled, not built
+          sed -i '/container_name: isso-client/{n;N;N;d}' docker-compose.yml
+
+      - name: Pull testbed from ${{ env.TESTBED_IMAGE }}
+        run: docker pull ${{ env.TESTBED_IMAGE }}
+
+      - name: Tag testbed image for local use
+        run: docker tag ${{ env.TESTBED_IMAGE }} isso-js-testbed:latest
+
+      - name: Build the stack
+        run: docker compose build isso-server
+
+      - name: Bring up containers
+        run: docker compose up -d
+
+      - name: Wait for isso-server container to be ready
+        run: 'for i in $(seq 1 30); do [ "$(docker inspect -f {{.State.Health.Status}} isso-server)" = "healthy" ] && s=0 && break || s=$? && sleep 0.3; done; (exit $s)'
+
+      - name: Update screenshots
+        run: make docker-update-screenshots
+
+      - name: Check for changes
+        id: check-changes
+        run: |
+          if git diff --quiet isso/js/tests/screenshots/reference/; then
+            echo "changes=false" >> $GITHUB_OUTPUT
+          else
+            echo "changes=true" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Create Pull Request
+        if: steps.check-changes.outputs.changes == 'true'
+        uses: peter-evans/create-pull-request@v5
+        with:
+          commit-message: "Update reference screenshots"
+          title: "Update reference screenshots"
+          body: |
+            This PR updates the reference screenshots generated by the automated workflow.
+
+            Please review the changes to ensure they are expected.
+          branch: automated-screenshot-update
+          delete-branch: true