feat: storefront subscription access guard + module-driven nav + URL rename

Add StorefrontAccessMiddleware that blocks storefront access for stores
without an active subscription, returning a multilingual unavailable page
(en/fr/de/lb) for page requests and JSON 403 for API requests. Multi-platform
aware: resolves subscription for detected platform with fallback to primary.

Also includes yesterday's session work:
- Module-driven storefront navigation via FrontendType.STOREFRONT menu declarations
- shop/ → storefront/ URL rename across 30+ templates
- Subscription context (tier_code) passed to storefront templates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/storefront/errors/base.html

@@ -8,7 +8,7 @@
     <title>{% block title %}{{ status_code }} - {{ status_name }}{% endblock %}{% if store %} | {{ store.name }}{% endif %}</title>
 
     {# Tailwind CSS #}
-    <link rel="stylesheet" href="{{ url_for('static', path='shop/css/tailwind.output.css') }}">
+    <link rel="stylesheet" href="{{ url_for('static', path='storefront/css/tailwind.output.css') }}">
 
     {# Store theme colors via CSS variables #}
     <style>
@@ -76,11 +76,11 @@
         {# Action Buttons #}
         <div class="flex gap-4 justify-center flex-wrap mt-8">
             {% block action_buttons %}
-            <a href="{{ base_url }}shop/"
+            <a href="{{ base_url }}storefront/"
                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">
                 Continue Shopping
             </a>
-            <a href="{{ base_url }}shop/contact"
+            <a href="{{ base_url }}storefront/contact"
                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">
                 Contact Us
             </a>
@@ -92,7 +92,7 @@
         {# Support Link #}
         <div class="mt-10 pt-8 border-t border-gray-200 text-sm text-gray-500">
             {% block support_link %}
-            Need help? <a href="{{ base_url }}shop/contact" class="text-theme-primary font-semibold hover:underline">Contact our support team</a>
+            Need help? <a href="{{ base_url }}storefront/contact" class="text-theme-primary font-semibold hover:underline">Contact our support team</a>
             {% endblock %}
         </div>