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/useConversations.ts

@@ -0,0 +1,47 @@
+/**
+ * React Query hooks for assistant conversation management
+ */
+
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import * as api from '../lib/api'
+
+/**
+ * List all conversations for a project
+ */
+export function useConversations(projectName: string | null) {
+  return useQuery({
+    queryKey: ['conversations', projectName],
+    queryFn: () => api.listAssistantConversations(projectName!),
+    enabled: !!projectName,
+    staleTime: 30000, // Cache for 30 seconds
+  })
+}
+
+/**
+ * Get a single conversation with all its messages
+ */
+export function useConversation(projectName: string | null, conversationId: number | null) {
+  return useQuery({
+    queryKey: ['conversation', projectName, conversationId],
+    queryFn: () => api.getAssistantConversation(projectName!, conversationId!),
+    enabled: !!projectName && !!conversationId,
+  })
+}
+
+/**
+ * Delete a conversation
+ */
+export function useDeleteConversation(projectName: string) {
+  const queryClient = useQueryClient()
+
+  return useMutation({
+    mutationFn: (conversationId: number) =>
+      api.deleteAssistantConversation(projectName, conversationId),
+    onSuccess: (_, deletedId) => {
+      // Invalidate conversations list
+      queryClient.invalidateQueries({ queryKey: ['conversations', projectName] })
+      // Remove the specific conversation from cache
+      queryClient.removeQueries({ queryKey: ['conversation', projectName, deletedId] })
+    },
+  })
+}