Check templates with bin/rails herb:check in CI

Rails 8.2 compiles HTML templates through Herb in strict mode, so a
template that doesn't compile fails to render. herb:check boots the app,
which loads ruby-vips, hence libvips.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -48,6 +48,29 @@ jobs:
       - name: Lint code for consistent style
         run: bin/rubocop
 
+  herb:
+    name: Herb check
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Install packages
+        run: sudo apt-get update && sudo apt-get install --no-install-recommends -y libvips
+
+      - name: Checkout code
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@a0102e0972be65f351c307e2d64b9314a57c8073 # v1.324.0
+        with:
+          ruby-version: .ruby-version
+          bundler-cache: true
+
+      - name: Check that every HTML template compiles through Herb
+        run: bin/rails herb:check
+
   lint-actions:
     name: GitHub Actions audit
     runs-on: ubuntu-latest