feat: production routing support for subdomain and custom domain modes

Double-mount store routes at /store/* and /store/{store_code}/* so the
same handlers work in dev path-based, prod path-based, prod subdomain,
and prod custom-domain modes.  Wire StorePlatform.custom_subdomain into
StoreContextMiddleware for per-platform subdomain overrides.  Add admin
custom-domain management UI, fix stale /shop/ reset link, add
/merchants/ to reserved paths, and server-render window.STORE_CODE for
JS that previously parsed the URL.

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

app/templates/store/base.html

@@ -56,6 +56,7 @@
 
     <!-- 1.5: Store Configuration (resolved via PlatformSettingsService) -->
     <script>
+        window.STORE_CODE = '{{ store_code | default("") }}';
         window.STORE_CONFIG = {
             locale: '{{ storefront_locale }}',
             currency: '{{ storefront_currency }}',