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/errors/generic.html

@@ -12,7 +12,7 @@
 <div class="error-code">Error Code: {{ error_code }}</div>
 
 <div class="action-buttons">
-    <a href="/store/dashboard" class="btn btn-primary">Go to Dashboard</a>
+    <a href="/store/{{ store_code }}/dashboard" class="btn btn-primary">Go to Dashboard</a>
     <a href="javascript:history.back()" class="btn btn-secondary">Go Back</a>
 </div>
 
@@ -37,6 +37,6 @@
 {% endif %}
 
 <div class="support-link">
-    Need assistance? <a href="/store/support">Contact Support</a>
+    Need assistance? <a href="/store/{{ store_code }}/support">Contact Support</a>
 </div>
-{% endblock %}
+{% endblock %}