feat: 优化情景切换功能

frontend/src/App.css

@@ -1674,6 +1674,54 @@ body {
   line-height: 1.4;
 }
 
+/* ---- Scenario Hint Card (非空状态顶部提示) ---- */
+
+.chat-panel__scenario-hint {
+  display: flex;
+  justify-content: center;
+  padding: 12px 24px 0;
+}
+
+.scenario-hint-card {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  padding: 12px 16px;
+  border-radius: var(--radius-sm);
+  background: linear-gradient(135deg, rgba(59, 130, 246, 0.08) 0%, rgba(99, 102, 241, 0.08) 100%);
+  border: 1px solid rgba(59, 130, 246, 0.2);
+  max-width: 520px;
+  width: 100%;
+  box-shadow: 0 2px 8px rgba(59, 130, 246, 0.06);
+}
+
+.scenario-hint-card__icon {
+  font-size: 1.8rem;
+  flex-shrink: 0;
+  filter: drop-shadow(0 2px 4px rgba(0, 0, 0, 0.1));
+}
+
+.scenario-hint-card__text {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  flex: 1;
+}
+
+.scenario-hint-card__text strong {
+  font-size: 0.9rem;
+  font-weight: 600;
+  color: var(--color-text);
+  letter-spacing: 0.01em;
+}
+
+.scenario-hint-card__text p {
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+  line-height: 1.5;
+  margin: 0;
+}
+
 /* ---- Voice Input Button ---- */
 
 .chat-input__voice {

frontend/src/components/ChatPanel/index.tsx

@@ -106,6 +106,21 @@ export function ChatPanel({
   return (
     <div className="chat-panel">
       <div className="chat-panel__messages" ref={containerRef}>
+        {/* 情景提示卡片（非空状态 + 非自由对话） */}
+        {messages.length > 0 && !isFreeChat && (
+          <div className="chat-panel__scenario-hint">
+            <div className="scenario-hint-card">
+              <span className="scenario-hint-card__icon">
+                {scenarios.find(s => s.id === activeScenario)?.icon}
+              </span>
+              <div className="scenario-hint-card__text">
+                <strong>{t(scenarios.find(s => s.id === activeScenario)?.nameKey || "")}</strong>
+                <p>{t(`scenario.${activeScenario}.hint`)}</p>
+              </div>
+            </div>
+          </div>
+        )}
+
         {/* 空状态：情景选择 + 场景卡片 */}
         {isEmpty && (
           <div className="chat-panel__welcome">

frontend/src/hooks/useVisionSession.ts

@@ -130,6 +130,7 @@ export function useVisionSession(accessToken?: string | null, conversationId?: s
           tts_enabled: config.ttsEnabled,
           detail_level: config.detailLevel,
           language: config.language,
+          scenario: config.scenario,
         },
       });
 
@@ -169,6 +170,7 @@ export function useVisionSession(accessToken?: string | null, conversationId?: s
             tts_enabled: next.ttsEnabled,
             detail_level: next.detailLevel,
             language: next.language,
+            scenario: next.scenario,
           },
         });
       }

frontend/src/lib/i18n/en-US.ts

@@ -33,14 +33,19 @@ export const enUS: TranslationMap = {
   "scenario.choose": "Choose a scenario",
   "scenario.freeChat": "Free Chat",
   "scenario.freeChat.desc": "AI visual assistant, chat freely",
+  "scenario.freeChat.hint": "AI will act as a visual assistant to answer your questions",
   "scenario.interviewer": "Mock Interviewer",
   "scenario.interviewer.desc": "AI plays interviewer for practice",
+  "scenario.interviewer.hint": "AI will act as an interviewer, asking professional questions and providing feedback",
   "scenario.englishTeacher": "English Tutor",
   "scenario.englishTeacher.desc": "AI English tutor, corrects grammar",
+  "scenario.englishTeacher.hint": "AI will converse in English, correct grammar mistakes, and guide deeper conversations",
   "scenario.debate": "Debate Opponent",
   "scenario.debate.desc": "AI challenges your viewpoints",
+  "scenario.debate.hint": "AI will argue against your viewpoints using logic and evidence",
   "scenario.interpreter": "Interpreter",
   "scenario.interpreter.desc": "Real-time Chinese-English translation",
+  "scenario.interpreter.hint": "AI will translate your speech in real-time (Chinese-English), without explanations",
 
   // Video indicators
   "video.observing": "👁️ Observing",

frontend/src/lib/i18n/ja-JP.ts

@@ -33,14 +33,19 @@ export const jaJP: TranslationMap = {
   "scenario.choose": "シナリオを選択",
   "scenario.freeChat": "フリーチャット",
   "scenario.freeChat.desc": "AIビジュアルアシスタントと自由に会話",
+  "scenario.freeChat.hint": "AIがビジュアルアシスタントとしてあなたの質問に答えます",
   "scenario.interviewer": "模擬面接官",
   "scenario.interviewer.desc": "AIが面接官として模擬面接を実施",
+  "scenario.interviewer.hint": "AIが面接官として専門的な質問をし、フィードバックを提供します",
   "scenario.englishTeacher": "英語の先生",
   "scenario.englishTeacher.desc": "AI英語教師が文法を修正し会話を導く",
+  "scenario.englishTeacher.hint": "AIが英語で会話し、文法の誤りを修正して深い対話を導きます",
   "scenario.debate": "ディベート相手",
   "scenario.debate.desc": "AIがあなたの观点に反論し論理的思考を鍛える",
+  "scenario.debate.hint": "AIが反対の立場から論理と証拠であなたの观点に反論します",
   "scenario.interpreter": "同時通訳",
   "scenario.interpreter.desc": "リアルタイムで中日相互翻訳",
+  "scenario.interpreter.hint": "AIがあなたの発言をリアルタイムで翻訳します（中日相互翻訳、説明なし）",
 
   // Video indicators
   "video.observing": "👁️ 観察中",

frontend/src/lib/i18n/zh-CN.ts

@@ -33,14 +33,19 @@ export const zhCN: TranslationMap = {
   "scenario.choose": "选择对话情景",
   "scenario.freeChat": "自由对话",
   "scenario.freeChat.desc": "AI 视觉助手，自由聊天",
+  "scenario.freeChat.hint": "AI 会作为视觉助手回答你的问题",
   "scenario.interviewer": "模拟面试官",
   "scenario.interviewer.desc": "AI 扮演面试官，进行模拟面试",
+  "scenario.interviewer.hint": "AI 会扮演面试官，逐步提出专业问题并点评你的回答",
   "scenario.englishTeacher": "英语老师",
   "scenario.englishTeacher.desc": "AI 英语外教，纠正语法并引导对话",
+  "scenario.englishTeacher.hint": "AI 会用英语对话，纠正语法错误并引导深入交流",
   "scenario.debate": "辩论对手",
   "scenario.debate.desc": "AI 反驳你的观点，锻炼逻辑思维",
+  "scenario.debate.hint": "AI 会站在反方立场，用逻辑和证据反驳你的观点",
   "scenario.interpreter": "同声翻译",
   "scenario.interpreter.desc": "实时中英互译，口语化翻译",
+  "scenario.interpreter.hint": "AI 会实时翻译你的话（中英互译），无解释评论",
 
   // Video indicators
   "video.observing": "👁️ 观察中",