fix(chat): preserve messages array during chat update

Object.assign was overwriting the existing messages array before the
ternary fallback could reference it. Capture the reference beforehand.

frontend/src/components/chatManager.ts

@@ -152,10 +152,13 @@ Alpine.data('chatManager', () => ({
         chat = { ...chunk.chat, messages: chunk.chat.messages || [] };
         this.chats.unshift(chat);
       } else {
+        // Preserve Messages - Object.assign would overwrite the existing
+        // messages array before we can check whether the chunk has any.
+        const existingMessages = chat.messages;
         Object.assign(chat, chunk.chat);
         chat.messages = chunk.chat.messages?.length
           ? chunk.chat.messages
-          : chat.messages;
+          : existingMessages;
       }
       this.selectedChatID = chunk.chat.id;
       this.updateHash(chunk.chat.id);