docs: update documentation for platform-aware storefront routing

Update 8 documentation files to reflect new URL scheme:
- Dev: /platforms/{code}/storefront/{store_code}/
- Prod: subdomain.platform.lu/ (root path = storefront)
- Rename DEFAULT_PLATFORM_CODE to MAIN_PLATFORM_CODE
- Replace hardcoded platform_id=1 with dynamic values
- Update route examples, middleware descriptions, code samples

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture/menu-management.md

@@ -176,7 +176,7 @@ from app.modules.core.services.menu_discovery_service import menu_discovery_serv
 sections = menu_discovery_service.get_menu_for_frontend(
     db=db,
     frontend_type=FrontendType.ADMIN,
-    platform_id=1,
+    platform_id=platform.id,
     user_id=current_user.id,
     is_super_admin=current_user.is_super_admin,
 )
@@ -185,7 +185,7 @@ sections = menu_discovery_service.get_menu_for_frontend(
 all_items = menu_discovery_service.get_all_menu_items(
     db=db,
     frontend_type=FrontendType.ADMIN,
-    platform_id=1,
+    platform_id=platform.id,
 )
 ```
 
@@ -285,7 +285,7 @@ from app.modules.enums import FrontendType
 # Platform "OMS" hides inventory from admin panel
 AdminMenuConfig(
     frontend_type=FrontendType.ADMIN,
-    platform_id=1,
+    platform_id=oms_platform.id,  # OMS platform
     menu_item_id="inventory",
     is_visible=False
 )
@@ -293,7 +293,7 @@ AdminMenuConfig(
 # Platform "OMS" hides letzshop from store dashboard
 AdminMenuConfig(
     frontend_type=FrontendType.STORE,
-    platform_id=1,
+    platform_id=oms_platform.id,  # OMS platform
     menu_item_id="letzshop",
     is_visible=False
 )