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/base.html

@@ -185,7 +185,7 @@
 
         <div class="action-buttons">
             {% block 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>
             {% endblock %}
         </div>
@@ -214,10 +214,10 @@
 
         <div class="support-link">
             {% block support_link %}
-            Need help? <a href="/store/support">Contact Store Support</a>
+            Need help? <a href="/store/{{ store_code }}/support">Contact Store Support</a>
             {% endblock %}
         </div>
         {% endblock %}
     </div>
 </body>
-</html>
+</html>