fix: display image attachments in user message bubbles

Images sent by the user were transmitted to the gateway correctly but
not shown in the chat UI, making it appear as if attachments were lost.

Now image blocks are included in the user message for immediate visual
feedback.

Closes #14

src/hooks/useGateway.ts

@@ -427,12 +427,15 @@ export function useGateway() {
 
   const sendMessage = useCallback(async (text: string, attachments?: Array<{ mimeType: string; fileName: string; content: string }>) => {
     const msgId = 'user-' + Date.now();
+    const imageBlocks: MessageBlock[] = (attachments ?? [])
+      .filter(a => a.mimeType.startsWith('image/'))
+      .map(a => ({ type: 'image' as const, mediaType: a.mimeType, data: a.content }));
     const userMsg: ChatMessage = {
       id: msgId,
       role: 'user',
       content: text,
       timestamp: Date.now(),
-      blocks: [{ type: 'text', text }],
+      blocks: [...imageBlocks, { type: 'text', text }],
       sendStatus: 'sending',
     };
     setMessages(prev => [...prev, userMsg]);