Installs and customizes assistant-ui elements, the styled shadcn-style component catalog served from the r.assistant-ui.com registry through `npx assistant-ui@latest add <item>`. Use when adding a prebuilt chat surface or widget (Thread, ThreadList, AssistantModal, AssistantSidebar, ToolFallback, ToolGroup, MarkdownText, Reasoning, Sources, Attachment, ModelSelector, Voice orb, McpConfig) or one of the 120 standalone elements (approval card, agent plan, code diff, data table, chart, and so on), choosing between runtime-connected `<name>.aui.tsx` files and props-driven standalone files, overriding Thread slots through the `components` prop, editing the copied source under `components/assistant-ui/elements/`, or picking the Radix versus Base UI flavor in `components.json`. Route here when an import from `@/components/assistant-ui/...` fails, an element renders unstyled, or the CLI installs the wrong flavor. For unstyled building blocks use primitives; for the CLI scaffold itself use setup.