fix(#338): Fix useChat stale messages with functional state updates

- Add messagesRef to track current messages and prevent stale closures
- Use functional updates for all setMessages calls
- Remove messages from sendMessage dependency array
- Add comprehensive tests verifying rapid sends don't lose messages

Refs #338

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/hooks/useChat.ts

@@ -73,6 +73,10 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
   const projectIdRef = useRef<string | null>(projectId ?? null);
   projectIdRef.current = projectId ?? null;
 
+  // Track messages in ref to prevent stale closures during rapid sends
+  const messagesRef = useRef<Message[]>(messages);
+  messagesRef.current = messages;
+
   /**
    * Convert our Message format to API ChatMessage format
    */
@@ -156,15 +160,19 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
         createdAt: new Date().toISOString(),
       };
 
-      // Add user message immediately
-      setMessages((prev) => [...prev, userMessage]);
+      // Add user message immediately using functional update
+      setMessages((prev) => {
+        const updated = [...prev, userMessage];
+        messagesRef.current = updated;
+        return updated;
+      });
       setIsLoading(true);
       setError(null);
 
       try {
-        // Prepare API request
-        const updatedMessages = [...messages, userMessage];
-        const apiMessages = convertToApiMessages(updatedMessages);
+        // Prepare API request - use ref to get current messages (prevents stale closure)
+        const currentMessages = messagesRef.current;
+        const apiMessages = convertToApiMessages(currentMessages);
 
         const request = {
           model,
@@ -189,9 +197,13 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
           totalTokens: (response.promptEvalCount ?? 0) + (response.evalCount ?? 0),
         };
 
-        // Add assistant message
-        const finalMessages = [...updatedMessages, assistantMessage];
-        setMessages(finalMessages);
+        // Add assistant message using functional update
+        let finalMessages: Message[] = [];
+        setMessages((prev) => {
+          finalMessages = [...prev, assistantMessage];
+          messagesRef.current = finalMessages;
+          return finalMessages;
+        });
 
         // Generate title from first user message if this is a new conversation
         const isFirstMessage =
@@ -220,7 +232,6 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
       }
     },
     [
-      messages,
       isLoading,
       conversationId,
       conversationTitle,