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/modules/billing/static/shared/js/feature-store.js

@@ -178,10 +178,11 @@
         },
 
         /**
-         * Get store code from URL
+         * Get store code from server-rendered value or URL fallback
          * @returns {string|null}
          */
         getStoreCode() {
+            if (window.STORE_CODE) return window.STORE_CODE;
             const path = window.location.pathname;
             const segments = path.split('/').filter(Boolean);
             // Direct: /store/{code}/...

app/modules/billing/static/shared/js/upgrade-prompts.js

@@ -139,9 +139,10 @@
         },
 
         /**
-         * Get store code from URL
+         * Get store code from server-rendered value or URL fallback
          */
         getStoreCode() {
+            if (window.STORE_CODE) return window.STORE_CODE;
             const path = window.location.pathname;
             const segments = path.split('/').filter(Boolean);
             if (segments[0] === 'store' && segments[1]) {