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>

main.py

@@ -417,13 +417,18 @@ for route_info in merchant_page_routes:
 # =============================================================================
 # STORE PAGES
 # =============================================================================
+# Store dashboard pages - Register at TWO prefixes:
+# 1. /store/* (for prod: subdomain/custom domain, after path rewrite by middleware)
+# 2. /store/{store_code}/* (for dev: path-based, after /platforms/{code}/ strip)
 logger.info("Auto-discovering store page routes...")
 store_page_routes = get_store_page_routes()
 logger.info(f"  Found {len(store_page_routes)} store page route modules")
 
+# Mount 1: /store/* (production subdomain/custom domain — store resolved by middleware)
+logger.info("  Registering store routes at /store/*")
 for route_info in store_page_routes:
     prefix = f"/store{route_info.custom_prefix}" if route_info.custom_prefix else "/store"
-    logger.info(f"  Registering {route_info.module_code} store pages at {prefix} (priority={route_info.priority})")
+    logger.info(f"    - {route_info.module_code} at {prefix} (priority={route_info.priority})")
     app.include_router(
         route_info.router,
         prefix=prefix,
@@ -431,6 +436,17 @@ for route_info in store_page_routes:
         include_in_schema=route_info.include_in_schema,
     )
 
+# Mount 2: /store/{store_code}/* (dev path-based + production path-based demo/trial)
+logger.info("  Registering store routes at /store/{store_code}/*")
+for route_info in store_page_routes:
+    prefix = f"/store/{{store_code}}{route_info.custom_prefix}" if route_info.custom_prefix else "/store/{store_code}"
+    app.include_router(
+        route_info.router,
+        prefix=prefix,
+        tags=route_info.tags,
+        include_in_schema=False,
+    )
+
 # =============================================================================
 # STOREFRONT PAGES (Customer Shop)
 # =============================================================================