feat: loyalty feature provider, admin data fixes, storefront mobile menu

- Add LoyaltyFeatureProvider with 11 BINARY/MERCHANT features for billing
  feature gating, wired into loyalty module definition
- Fix subscription-tiers admin page showing 0 features by populating
  feature_codes from tier relationship in all admin tier endpoints
- Fix merchants admin page showing 0 stores and N/A owner by adding
  store_count and owner_email to MerchantResponse and eager-loading owner
- Add "no tiers" warning with link in subscription creation modal when
  platform has no configured tiers
- Add missing mobile menu panel to storefront base template so hamburger
  toggle actually shows navigation links

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

app/templates/storefront/base.html

@@ -206,6 +206,36 @@
         </div>
     </header>
 
+    {# Mobile menu panel #}
+    <div x-show="mobileMenuOpen"
+         x-cloak
+         x-transition:enter="transition ease-out duration-200"
+         x-transition:enter-start="opacity-0 -translate-y-2"
+         x-transition:enter-end="opacity-100 translate-y-0"
+         x-transition:leave="transition ease-in duration-150"
+         x-transition:leave-start="opacity-100 translate-y-0"
+         x-transition:leave-end="opacity-0 -translate-y-2"
+         class="md:hidden bg-white dark:bg-gray-900 border-b border-gray-200 dark:border-gray-700 shadow-lg">
+        <nav class="max-w-7xl mx-auto px-4 py-4 space-y-1">
+            <a href="{{ base_url }}" @click="closeMobileMenu()"
+               class="block px-3 py-2 rounded-lg text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 font-medium">
+                Home
+            </a>
+            {% for item in storefront_nav.get('nav', []) %}
+            <a href="{{ base_url }}{{ item.route }}" @click="closeMobileMenu()"
+               class="block px-3 py-2 rounded-lg text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700">
+                {{ _(item.label_key) }}
+            </a>
+            {% endfor %}
+            {% for page in header_pages|default([]) %}
+            <a href="{{ base_url }}storefront/{{ page.slug }}" @click="closeMobileMenu()"
+               class="block px-3 py-2 rounded-lg text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700">
+                {{ page.title }}
+            </a>
+            {% endfor %}
+        </nav>
+    </div>
+
     {# Main Content Area #}
     <main class="min-h-screen">
         {% block content %}