fix: address performance and code quality issues in conversation history

Performance improvements:
- Fix N+1 query in get_conversations() using COUNT subquery instead of
  len(c.messages) which triggered lazy loading for each conversation
- Add SQLAlchemy engine caching to avoid creating new database connections
  on every request
- Add React.memo to ChatMessage component to prevent unnecessary re-renders
  during message streaming
- Move BOLD_REGEX to module scope to avoid recreating on each render

Code quality improvements:
- Remove 10+ console.log debug statements from AssistantChat.tsx and
  AssistantPanel.tsx that were left from development
- Add user feedback for delete errors in ConversationHistory - dialog now
  stays open and shows error message instead of silently failing
- Update ConfirmDialog to accept ReactNode for message prop to support
  rich error content

These changes address issues identified in the code review of PR #74
(conversation history feature).

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

ui/src/components/AssistantPanel.tsx

@@ -62,13 +62,6 @@ export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelP
     timestamp: msg.timestamp ? new Date(msg.timestamp) : new Date(),
   }))
 
-  console.log('[AssistantPanel] State:', {
-    conversationId,
-    isLoadingConversation,
-    conversationDetailId: conversationDetail?.id,
-    initialMessagesCount: initialMessages?.length ?? 0
-  })
-
   // Persist conversation ID changes to localStorage
   useEffect(() => {
     setStoredConversationId(projectName, conversationId)
@@ -86,7 +79,6 @@ export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelP
 
   // Handle selecting a conversation from history
   const handleSelectConversation = useCallback((id: number) => {
-    console.log('[AssistantPanel] handleSelectConversation called with id:', id)
     setConversationId(id)
   }, [])