feat(auth): implement auto guest mode and enhance token system

- Add role-based token system with metadata support
- Implement automatic guest mode for unconfigured authentication
- Create new /auth-status endpoint for authentication status checking
- Modify frontend to auto-detect auth status and bypass login when appropriate
- Add guest mode indicator in site header for better UX

This change allows users to automatically access the system without manual
login when authentication is not configured, while maintaining secure
authentication when credentials are properly set up.

lightrag_webui/src/locales/en.json

@@ -28,7 +28,9 @@
     "loggingIn": "Logging in...",
     "successMessage": "Login succeeded",
     "errorEmptyFields": "Please enter your username and password",
-    "errorInvalidCredentials": "Login failed, please check username and password"
+    "errorInvalidCredentials": "Login failed, please check username and password",
+    "authDisabled": "Authentication is disabled. Using guest access.",
+    "guestMode": "Guest Mode"
   },
   "documentPanel": {
     "clearDocuments": {

lightrag_webui/src/locales/zh.json

@@ -28,7 +28,9 @@
     "loggingIn": "登录中...",
     "successMessage": "登录成功",
     "errorEmptyFields": "请输入您的用户名和密码",
-    "errorInvalidCredentials": "登录失败，请检查用户名和密码"
+    "errorInvalidCredentials": "登录失败，请检查用户名和密码",
+    "authDisabled": "认证已禁用，使用访客访问模式。",
+    "guestMode": "访客模式"
   },
   "documentPanel": {
     "clearDocuments": {