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/ChatInput.tsx

@@ -15,7 +15,7 @@ export interface ImageAttachment {
   preview: string;
 }
 
-export function ChatInput({ onSend, disabled, connecting }: { onSend: (content: string, images?: Array<{ data: string; mimeType: string }>) => void; disabled?: boolean; connecting?: boolean }) {
+export function ChatInput({ onSend, disabled, connecting }: { onSend: (content: string, images?: Array<{ type: 'image'; data: string; mimeType: string }>) => void; disabled?: boolean; connecting?: boolean }) {
   const [input, setInput] = useState('');
   const [showCmds, setShowCmds] = useState(false);
   const [cmdIdx, setCmdIdx] = useState(0);