feat：添加对话情景功能

frontend/src/types/index.ts

@@ -9,6 +9,7 @@ export interface SessionConfig {
   ttsEnabled: boolean;
   detailLevel: "low" | "high";
   language: string;
+  scenario: string;    // 情景 ID，如 "free_chat"、"interviewer"
 }
 
 export type Theme = "dark" | "light";
@@ -32,7 +33,7 @@ export interface SessionSummary {
 // ---- 聊天消息 ----
 
 export interface ChatMessage {
-  role: "user" | "assistant";
+  role: "user" | "assistant" | "system";
   content: string;
   imageUrl?: string;
   timestamp: number;