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/core/static/store/js/init-alpine.js

@@ -74,8 +74,9 @@ function data() {
                 ? segments[segments.length - 2]
                 : last;
 
-            // Get store code from URL
-            if (segments[0] === 'store' && segments[1]) {
+            // Get store code from server-rendered value or URL fallback
+            this.storeCode = window.STORE_CODE || null;
+            if (!this.storeCode && segments[0] === 'store' && segments[1]) {
                 this.storeCode = segments[1];
             }
 
@@ -281,14 +282,18 @@ function emailSettingsWarning() {
         storeCode: null,
 
         async init() {
-            // Get store code from URL
-            const path = window.location.pathname;
-            const segments = path.split('/').filter(Boolean);
-            if (segments[0] === 'store' && segments[1]) {
-                this.storeCode = segments[1];
+            // Get store code from server-rendered value or URL fallback
+            this.storeCode = window.STORE_CODE || null;
+            if (!this.storeCode) {
+                const path = window.location.pathname;
+                const segments = path.split('/').filter(Boolean);
+                if (segments[0] === 'store' && segments[1]) {
+                    this.storeCode = segments[1];
+                }
             }
 
             // Skip if we're on the settings page (to avoid showing banner on config page)
+            const path = window.location.pathname;
             if (path.includes('/settings')) {
                 this.loading = false;
                 return;