fix: 修复无摄像头/麦克风时聊天框不显示的问题

- 重构 ChatPanel 逻辑，连接后始终显示输入框
- 将欢迎消息移入消息容器内
- 确保输入框在任何状态下都可见

frontend/src/App.css

@@ -493,6 +493,32 @@ body {
   gap: 12px;
 }
 
+.chat-panel__welcome {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  flex: 1;
+  gap: 12px;
+  color: var(--color-text-muted);
+  font-size: 0.85rem;
+  padding: 40px 0;
+}
+
+.chat-panel__welcome-icon {
+  font-size: 2.4rem;
+  opacity: 0.12;
+  margin-bottom: 4px;
+}
+
+.chat-panel__welcome-hint {
+  font-size: 0.75rem;
+  opacity: 0.6;
+  max-width: 240px;
+  text-align: center;
+  line-height: 1.5;
+}
+
 .chat-panel--empty {
   display: flex;
   flex-direction: column;