feat(chat): 为图片附件添加类型标识以支持多类型附件

在图片消息的接口类型中显式添加 `type: 'image'` 属性，为后续支持多种附件类型（如视频、文件等）做好类型基础。

web/client/src/components/chat/ChatView.tsx

@@ -108,7 +108,7 @@ export function ChatView() {
   }, [removeMessage]);
 
   // Delayed conversation creation
-  const handleSendNew = useCallback(async (content: string, images?: Array<{ data: string; mimeType: string }>) => {
+  const handleSendNew = useCallback(async (content: string, images?: Array<{ type: 'image'; data: string; mimeType: string }>) => {
     if (creatingRef.current) return;
     creatingRef.current = true;
     setPendingMessage({ content, images });
@@ -126,7 +126,7 @@ export function ChatView() {
     creatingRef.current = false;
   }, [createConversation, selectedAccountId]);
 
-  const handleSend = useCallback((content: string, images?: Array<{ data: string; mimeType: string }>) => {
+  const handleSend = useCallback((content: string, images?: Array<{ type: 'image'; data: string; mimeType: string }>) => {
     if (quote) { content = `> ${quote}\n\n${content}`; setQuote(null); }
     if (conversationId) {
       send(content, images);