refactor: rename shop to storefront for consistency

Rename all "shop" directories and references to "storefront" to match
the API and route naming convention already in use.

Renamed directories:
- app/templates/shop/ → app/templates/storefront/
- static/shop/ → static/storefront/
- app/templates/shared/macros/shop/ → .../macros/storefront/
- docs/frontend/shop/ → docs/frontend/storefront/

Renamed files:
- shop.css → storefront.css
- shop-layout.js → storefront-layout.js

Updated references in:
- app/routes/storefront_pages.py (21 template references)
- app/modules/cms/routes/pages/vendor.py
- app/templates/storefront/base.html (static paths)
- All storefront templates (extends/includes)
- docs/architecture/frontend-structure.md

This aligns the template/static naming with:
- Route file: storefront_pages.py
- API directory: app/api/v1/storefront/
- Module routes: */routes/api/storefront.py
- URL paths: /storefront/*

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

app/templates/storefront/errors/429.html

@@ -0,0 +1,32 @@
+{# app/templates/storefront/errors/429.html #}
+{# 429 Too Many Requests error page - rate limiting #}
+{% extends "storefront/errors/base.html" %}
+
+{% block icon %}⏱️{% endblock %}
+
+{% block title %}429 - Please Slow Down{% endblock %}
+
+{% block extra_content %}
+{% if details and details.retry_after %}
+<div class="my-6 p-4 bg-amber-50 rounded-xl">
+    <p class="text-amber-800 font-semibold">
+        Please wait {{ details.retry_after }} seconds
+    </p>
+</div>
+{% endif %}
+{% endblock %}
+
+{% block action_buttons %}
+<a href="javascript:location.reload()"
+   class="inline-flex items-center px-8 py-4 rounded-xl font-semibold text-white bg-theme-primary hover:opacity-90 hover:-translate-y-0.5 transition-all shadow-lg">
+    Try Again
+</a>
+<a href="{{ base_url }}shop/"
+   class="inline-flex items-center px-8 py-4 rounded-xl font-semibold text-theme-primary border-2 border-theme-primary hover:bg-theme-primary hover:text-white hover:-translate-y-0.5 transition-all">
+    Go to Home
+</a>
+{% endblock %}
+
+{% block support_link %}
+Questions? <a href="{{ base_url }}shop/contact" class="text-theme-primary font-semibold hover:underline">Contact us</a>
+{% endblock %}