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/401.html

@@ -14,8 +14,8 @@
 <div class="error-code">Error Code: {{ error_code }}</div>
 
 <div class="action-buttons">
-    <a href="/store/login" class="btn btn-primary">Log In</a>
-    <a href="/store/dashboard" class="btn btn-secondary">Dashboard</a>
+    <a href="/store/{{ store_code }}/login" class="btn btn-primary">Log In</a>
+    <a href="/store/{{ store_code }}/dashboard" class="btn btn-secondary">Dashboard</a>
 </div>
 
 {% if show_debug %}
@@ -39,6 +39,6 @@
 {% endif %}
 
 <div class="support-link">
-    Having login issues? <a href="/store/support">Contact Support</a>
+    Having login issues? <a href="/store/{{ store_code }}/support">Contact Support</a>
 </div>
-{% endblock %}
+{% endblock %}