🐛 fix(locales): Update invite code text to indicate it's optional

apps/admin/locales/en-US/auth.json

@@ -28,7 +28,7 @@
     "email": "Please enter a valid email address.",
     "existingAccount": "Already have an account?",
     "get": "Get",
-    "invite": "Invitation code",
+    "invite": "Invitation code (optional)",
     "message": "#### Dear User, Hello!\n\nThank you for your attention and support. Due to adjustments in site operation strategy, we have closed the new user registration function. During this period, existing users will not be affected.\n\nWe are committed to providing you with better service and experience, so we will conduct comprehensive system optimization and feature upgrades during the registration closure. In the future, we will welcome you with better content and services.\n\nPlease follow our website and social media platforms to get the latest updates and notifications. Thank you for your understanding and support.\n\nIf you have any questions or need assistance, please feel free to contact our customer service team.\n\n**Thank you again for your support and understanding.**",
     "passwordMismatch": "Passwords do not match",
     "success": "Registration successful, automatically logged in!",