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/App.css

@@ -119,10 +119,10 @@ body {
 
 .chat-section {
   flex: 1;
-  overflow-y: auto;
   display: flex;
   flex-direction: column;
   gap: 12px;
+  overflow: hidden;
 }
 
 /* ---- Video Preview ---- */
@@ -157,6 +157,8 @@ body {
   display: flex;
   flex-direction: column;
   gap: 12px;
+  overflow-y: auto;
+  flex: 1;
 }
 
 .chat-panel--empty {
@@ -248,3 +250,111 @@ body {
 .btn--warning:hover {
   opacity: 0.9;
 }
+
+/* ---- Streaming Cursor ---- */
+
+.cursor {
+  display: inline;
+  animation: blink 0.8s step-end infinite;
+  color: var(--color-success);
+}
+
+@keyframes blink {
+  0%, 100% { opacity: 1; }
+  50% { opacity: 0; }
+}
+
+/* ---- Message Meta ---- */
+
+.chat-message__meta {
+  margin-top: 6px;
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+}
+
+/* ---- System Message ---- */
+
+.system-message {
+  text-align: center;
+  padding: 8px 16px;
+  border-radius: var(--radius);
+  font-size: 0.85rem;
+}
+
+.system-message--warning {
+  background: rgba(245, 158, 11, 0.15);
+  color: var(--color-warning);
+  border: 1px solid rgba(245, 158, 11, 0.3);
+}
+
+/* ---- Toast ---- */
+
+.toast-container {
+  position: fixed;
+  top: 16px;
+  right: 16px;
+  z-index: 1000;
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+
+.toast {
+  padding: 10px 16px;
+  border-radius: var(--radius);
+  font-size: 0.85rem;
+  cursor: pointer;
+  animation: slideIn 0.3s ease-out;
+  max-width: 320px;
+}
+
+.toast--error {
+  background: rgba(239, 68, 68, 0.9);
+  color: white;
+}
+
+.toast--warning {
+  background: rgba(245, 158, 11, 0.9);
+  color: #000;
+}
+
+.toast--info {
+  background: rgba(37, 99, 235, 0.9);
+  color: white;
+}
+
+@keyframes slideIn {
+  from {
+    opacity: 0;
+    transform: translateX(20px);
+  }
+  to {
+    opacity: 1;
+    transform: translateX(0);
+  }
+}
+
+/* ---- Video Overlay ---- */
+
+.video-overlay {
+  position: absolute;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.3);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  border-radius: var(--radius);
+}
+
+.video-overlay__spinner {
+  width: 32px;
+  height: 32px;
+  border: 3px solid rgba(255, 255, 255, 0.3);
+  border-top-color: white;
+  border-radius: 50%;
+  animation: spin 0.8s linear infinite;
+}
+
+@keyframes spin {
+  to { transform: rotate(360deg); }
+}