feat: 支持无摄像头/麦克风模式打字聊天

- 修改 startSession 逻辑，摄像头和麦克风为可选
- 连接成功后始终显示聊天输入框
- 更新 UI 提示文字，引导用户打字对话

frontend/src/App.tsx

@@ -173,6 +173,16 @@ function App() {
                 <span>点击下方按钮开始对话</span>
               </div>
             )}
+            {isConnected && !isCameraOn && (
+              <div className="video-placeholder">
+                <svg className="video-placeholder__icon" width="48" height="48" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.2" strokeLinecap="round" strokeLinejoin="round">
+                  <path d="M23 19a2 2 0 0 1-2 2H3a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h4l2-3h6l2 3h4a2 2 0 0 1 2 2z" />
+                  <circle cx="12" cy="13" r="4" />
+                </svg>
+                <span>摄像头未开启</span>
+                <span className="video-placeholder__hint">可在右侧聊天框打字对话</span>
+              </div>
+            )}
           </div>
 
           {/* 视频下方控制栏 */}