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

server/routers/assistant_chat.py

@@ -269,18 +269,23 @@ async def assistant_chat_websocket(websocket: WebSocket, project_name: str):
                 elif msg_type == "start":
                     # Get optional conversation_id to resume
                     conversation_id = message.get("conversation_id")
+                    logger.info(f"Processing start message with conversation_id={conversation_id}")
 
                     try:
                         # Create a new session
+                        logger.info(f"Creating session for {project_name}")
                         session = await create_session(
                             project_name,
                             project_dir,
                             conversation_id=conversation_id,
                         )
+                        logger.info(f"Session created, starting...")
 
                         # Stream the initial greeting
                         async for chunk in session.start():
+                            logger.info(f"Sending chunk: {chunk.get('type')}")
                             await websocket.send_json(chunk)
+                        logger.info("Session start complete")
                     except Exception as e:
                         logger.exception(f"Error starting assistant session for {project_name}")
                         await websocket.send_json({