refactor: 删除实时分析/按需识别/纯聊天三种模式切换

三种模式仅为前端 UI 区分，后端无感知，实际使用中价值不大：
- 纯聊天模式名不副实（VAD 仍附带摄像头帧）
- 实时分析增加 API 成本且体验不佳
- 按需识别已是默认且最自然的交互方式

删除后保留按需识别行为：语音带图 + 手动识别按钮，UI 更简洁。

frontend/src/App.tsx

@@ -24,9 +24,6 @@ import type { Theme } from "./types";
 import type { UserScenario } from "./lib/api/scenarios";
 import "./App.css";
 
-/** AI 视觉模式 */
-type VisionMode = "realtime" | "ondemand" | "chat";
-
 /** 内部组件，确保在 I18nContext.Provider 内部使用 hooks */
 function AppContent() {
   const { isAuthenticated, isLoading, user, logout, accessToken } = useAuth();
@@ -36,7 +33,6 @@ function AppContent() {
   const [theme, setTheme] = useState<Theme>(loadTheme);
   const [elapsed, setElapsed] = useState(0);
   const [sidebarOpen, setSidebarOpen] = useState(false);
-  const [visionMode, setVisionMode] = useState<VisionMode>("ondemand");
   const timerRef = useRef<ReturnType<typeof setInterval> | null>(null);
 
   // ---- 自建情景管理 ----
@@ -91,8 +87,6 @@ function AppContent() {
     config,
     updateConfig,
     stats,
-    mode,
-    isObserving,
     startSession,
     stopSession,
     stopVideo,
@@ -365,13 +359,13 @@ function AppContent() {
               <div className="detail-badge">HD</div>
             )}
             {/* AI 视觉状态指示 */}
-            {isConnected && visionMode !== "chat" && (
-              <div className={`ai-vision-indicator ${isObserving ? "ai-vision-indicator--active" : ""}`}>
+            {isConnected && (
+              <div className="ai-vision-indicator">
                 <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
                   <path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z" />
                   <circle cx="12" cy="12" r="3" />
                 </svg>
-                <span>{isObserving ? tr("video.observing") : "AI"}</span>
+                <span>AI</span>
               </div>
             )}
             {isSpeaking && (
@@ -427,27 +421,6 @@ function AppContent() {
                     </select>
                   </div>
                 </div>
-                {/* 模式切换器 */}
-                <div className="video-controls__mode">
-                  <button
-                    className={`mode-btn ${visionMode === "realtime" ? "mode-btn--active" : ""}`}
-                    onClick={() => setVisionMode("realtime")}
-                  >
-                    {tr("controls.mode.realtime")}
-                  </button>
-                  <button
-                    className={`mode-btn ${visionMode === "ondemand" ? "mode-btn--active" : ""}`}
-                    onClick={() => setVisionMode("ondemand")}
-                  >
-                    {tr("controls.mode.ondemand")}
-                  </button>
-                  <button
-                    className={`mode-btn ${visionMode === "chat" ? "mode-btn--active" : ""}`}
-                    onClick={() => setVisionMode("chat")}
-                  >
-                    {tr("controls.mode.chat")}
-                  </button>
-                </div>
               </>
             ) : isCameraOn ? (
               <>
@@ -467,16 +440,14 @@ function AppContent() {
                   >
                     🎤
                   </button>
-                  {/* 识别画面按钮（按需模式下显示） */}
-                  {visionMode === "ondemand" && (
-                    <button
-                      className="btn--recognize"
-                      onClick={handleRecognize}
-                      disabled={isProcessing}
-                    >
-                      🔍 {tr("controls.recognize")}
-                    </button>
-                  )}
+                  {/* 识别画面按钮 */}
+                  <button
+                    className="btn--recognize"
+                    onClick={handleRecognize}
+                    disabled={isProcessing}
+                  >
+                    🔍 {tr("controls.recognize")}
+                  </button>
                   {isProcessing && (
                     <button className="btn btn--warning" onClick={interrupt}>
                       {tr("controls.interrupt")}
@@ -486,27 +457,6 @@ function AppContent() {
                     {tr("controls.stopVideo")}
                   </button>
                 </div>
-                {/* 通话态模式切换 */}
-                <div className="video-controls__mode">
-                  <button
-                    className={`mode-btn ${visionMode === "realtime" ? "mode-btn--active" : ""}`}
-                    onClick={() => setVisionMode("realtime")}
-                  >
-                    {tr("controls.mode.realtime")}
-                  </button>
-                  <button
-                    className={`mode-btn ${visionMode === "ondemand" ? "mode-btn--active" : ""}`}
-                    onClick={() => setVisionMode("ondemand")}
-                  >
-                    {tr("controls.mode.ondemand")}
-                  </button>
-                  <button
-                    className={`mode-btn ${visionMode === "chat" ? "mode-btn--active" : ""}`}
-                    onClick={() => setVisionMode("chat")}
-                  >
-                    {tr("controls.mode.chat")}
-                  </button>
-                </div>
               </>
             ) : (
               <>
@@ -547,9 +497,6 @@ function AppContent() {
                   </option>
                 ))}
               </select>
-              {isConnected && mode === "observation" && (
-                <span className="chat-panel-header__mode">{tr("chat.mode.observation")}</span>
-              )}
               {isConnected && stats.queryCount > 0 && (
                 <span className="chat-panel-header__stats">
                   {stats.queryCount} {tr("statusbar.recognitions")}