feat: 完善端到端对话闭环，增强消息渲染与错误处理

- ChatPanel：流式光标动画、token/延迟元数据、自动滚动、三种空状态
- Toast 组件：错误码映射为中文友好文案，3 秒自动消失
- useVisionSession：stt_result 流式更新、防重复发送、打断保存未完成内容、结束清理全部状态
- App：断线提示、视频 loading 遮罩、按钮连接中状态
- ChatMessage 增加 latencyMs、model 字段

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/Toast/index.tsx

@@ -0,0 +1,51 @@
+// ============================================================
+// Toast — 轻量通知组件
+// 职责：3 秒自动消失的通知提示
+// ============================================================
+
+import { useCallback, useEffect, useState } from "react";
+import { registerToastSetter, type ToastItem } from "../../lib/toast";
+
+export type { ToastItem };
+
+/** Toast 容器组件，放在 App 根部 */
+export function ToastContainer() {
+  const [toasts, setToasts] = useState<ToastItem[]>([]);
+
+  // 注册全局 setter
+  useEffect(() => {
+    registerToastSetter(setToasts);
+    return () => registerToastSetter(null);
+  }, []);
+
+  const dismiss = useCallback((id: number) => {
+    setToasts((prev) => prev.filter((t) => t.id !== id));
+  }, []);
+
+  return (
+    <div className="toast-container">
+      {toasts.map((t) => (
+        <ToastItemView key={t.id} item={t} onDismiss={dismiss} />
+      ))}
+    </div>
+  );
+}
+
+function ToastItemView({
+  item,
+  onDismiss,
+}: {
+  item: ToastItem;
+  onDismiss: (id: number) => void;
+}) {
+  useEffect(() => {
+    const timer = setTimeout(() => onDismiss(item.id), 3000);
+    return () => clearTimeout(timer);
+  }, [item.id, onDismiss]);
+
+  return (
+    <div className={`toast toast--${item.type}`} onClick={() => onDismiss(item.id)}>
+      {item.message}
+    </div>
+  );
+}