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/marketplace/static/store/js/onboarding.js

@@ -615,10 +615,10 @@ function storeOnboarding(initialLang = 'en') {
         async handleLogout() {
             onboardingLog.info('Logging out from onboarding...');
 
-            // Get store code from URL
+            // Get store code from server-rendered value or URL fallback
             const path = window.location.pathname;
             const segments = path.split('/').filter(Boolean);
-            const storeCode = segments[0] === 'store' && segments[1] ? segments[1] : '';
+            const storeCode = window.STORE_CODE || (segments[0] === 'store' && segments[1] ? segments[1] : '');
 
             try {
                 // Call logout API