fix: resolve billing page issues

- Add missing icons: x-mark, paint-brush, arrow-trending-up,
  puzzle-piece, arrow-down-tray, exclamation-triangle
- Fix billing.html to use correct block name (extra_scripts
  instead of scripts) so billing.js loads properly

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

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

app/templates/vendor/billing.html

@@ -446,6 +446,6 @@
 
 {% endblock %}
 
-{% block scripts %}
+{% block extra_scripts %}
 <script src="/static/vendor/js/billing.js"></script>
 {% endblock %}