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>

middleware/platform_context.py

@@ -410,7 +410,7 @@ class PlatformContextMiddleware:
                         _RESERVED = (
                             "/store/", "/admin/", "/api/", "/static/",
                             "/storefront/", "/health", "/docs", "/redoc",
-                            "/media/", "/assets/",
+                            "/media/", "/assets/", "/merchants/",
                         )
                         if not any(clean_path.startswith(p) for p in _RESERVED):
                             new_path = "/storefront" + clean_path