fix: use correct block name in messages template + add TPL-008 rule

- Fix messages.html: change {% block page_scripts %} to {% block extra_scripts %}
  (page_scripts doesn't exist in admin/base.html, causing JS not to load)

- Add TPL-008 architecture rule to catch invalid template block names
  This prevents silent failures where content in undefined blocks is ignored

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.architecture-rules/frontend.yaml

@@ -243,6 +243,35 @@ template_rules:
       file_pattern: "app/templates/**/*.html"
       recommended_pattern: '<template x-if="items.length === 0">No items</template>'
 
+  - id: "TPL-008"
+    name: "Use valid block names from base templates"
+    severity: "error"
+    description: |
+      Templates must use block names that exist in their base template.
+      Using undefined blocks silently fails (content is not rendered).
+
+      Admin base blocks: title, extra_head, alpine_data, content, extra_scripts
+      Vendor base blocks: title, extra_head, alpine_data, content, extra_scripts
+      Shop base blocks: title, description, extra_head, alpine_data, content, extra_scripts
+
+      WRONG: {% block page_scripts %}...{% endblock %}  (undefined)
+      RIGHT: {% block extra_scripts %}...{% endblock %}
+    pattern:
+      file_pattern: "app/templates/admin/**/*.html"
+      valid_blocks:
+        - "title"
+        - "extra_head"
+        - "alpine_data"
+        - "content"
+        - "extra_scripts"
+      forbidden_patterns:
+        - "{% block page_scripts %}"
+        - "{% block scripts %}"
+        - "{% block js %}"
+        - "{% block footer_scripts %}"
+      exceptions:
+        - "base.html"
+
 # ============================================================================
 # FRONTEND COMPONENT RULES
 # ============================================================================