feat: add conversation history feature to AI assistant

- Add ConversationHistory dropdown component with list of past conversations
- Add useConversations hook for fetching and managing conversations via React Query
- Implement conversation switching with proper state management
- Fix bug where reopening panel showed new greeting instead of resuming conversation
- Fix bug where selecting from history caused conversation ID to revert
- Add server-side history context loading for resumed conversations
- Add Playwright E2E tests for conversation history feature
- Add logging for debugging conversation flow

Key changes:
- AssistantPanel: manages conversation state with localStorage persistence
- AssistantChat: header with [+] New Chat and [History] buttons
- Server: skips greeting for resumed conversations, loads history context on first message
- Fixed race condition in onConversationCreated callback

ui/src/hooks/useAssistantChat.ts

@@ -120,6 +120,7 @@ export function useAssistantChat({
     ws.onmessage = (event) => {
       try {
         const data = JSON.parse(event.data) as AssistantChatServerMessage;
+        console.log('[useAssistantChat] Received WebSocket message:', data.type, data);
 
         switch (data.type) {
           case "text": {
@@ -277,6 +278,7 @@ export function useAssistantChat({
             payload.conversation_id = existingConversationId;
             setConversationId(existingConversationId);
           }
+          console.log('[useAssistantChat] Sending start message:', payload);
           wsRef.current.send(JSON.stringify(payload));
         } else if (wsRef.current?.readyState === WebSocket.CONNECTING) {
           checkAndSendTimeoutRef.current = window.setTimeout(checkAndSend, 100);
@@ -336,7 +338,7 @@ export function useAssistantChat({
 
   const clearMessages = useCallback(() => {
     setMessages([]);
-    setConversationId(null);
+    // Don't reset conversationId here - it will be set by start() when switching
   }, []);
 
   return {