feat：添加对话情景功能

frontend/src/lib/scenarios.ts

@@ -0,0 +1,47 @@
+// ============================================================
+// scenarios — 对话情景定义
+// 职责：定义预置情景的展示数据和交互行为
+// 注意：system prompt 由后端 scenarios.go 管理，此处仅定义 UI 展示数据
+// ============================================================
+
+export interface Scenario {
+  id: string;
+  icon: string;
+  nameKey: string;       // i18n key
+  descKey: string;       // i18n key
+  defaultLanguage?: string;  // 可选：切换情景时自动设置的语言
+}
+
+export const scenarios: Scenario[] = [
+  {
+    id: "free_chat",
+    icon: "💬",
+    nameKey: "scenario.freeChat",
+    descKey: "scenario.freeChat.desc",
+  },
+  {
+    id: "interviewer",
+    icon: "🎯",
+    nameKey: "scenario.interviewer",
+    descKey: "scenario.interviewer.desc",
+  },
+  {
+    id: "english_teacher",
+    icon: "📚",
+    nameKey: "scenario.englishTeacher",
+    descKey: "scenario.englishTeacher.desc",
+    defaultLanguage: "en-US",
+  },
+  {
+    id: "debate",
+    icon: "⚔️",
+    nameKey: "scenario.debate",
+    descKey: "scenario.debate.desc",
+  },
+  {
+    id: "interpreter",
+    icon: "🌐",
+    nameKey: "scenario.interpreter",
+    descKey: "scenario.interpreter.desc",
+  },
+];