- Add ChatConversationItem with DropdownMenu actions (Rename, Pin/Unpin, Archive, Delete) and active highlight
- Add ChatConversationList with IntersectionObserver infinite scroll, loading skeletons, delete confirmation dialog, and CRUD handlers
- Add ChatMessageList with auto-scroll-to-bottom on new messages and empty state
- Update ChatPanel to render ChatConversationList (left column) and ChatMessageList (right column); handleSend uses two paths: direct chatApi for new conversations, hook mutation for existing ones
