Build a page: Chat (ChatGPT-style assistant)

Route: /chat (client page — metadata in a sibling layout.tsx)
Nav: add to nav-config.ts under the "shadcn/ui" section.
Page frame: flex h-full flex-col -> action header with <Breadcrumbs/> ->
content min-h-0 flex-1 (two-pane).

Layout (clean, enterprise, ChatGPT-like):

- Left (w-64, bg-muted): "New chat" button + scrollable conversation history.
- Main: a centered (max-w-3xl) scrolling column of messages, then a composer.

Messages: avatar (You / AI) + role label + text; assistant vs user differ by
avatar color only. No colored bubbles — keep it clean.
Composer: rounded-2xl bordered box holding an auto-growing <textarea>
(Enter sends, Shift+Enter = newline), an attach (+) button (left),
and a primary send icon button (right). focus-within highlights it.

Attachments (images + files):

- Hidden <input type="file" multiple accept="image/*,.pdf,..."> triggered by +.
- Pending attachments show as chips above the textarea (image -> thumbnail,
  else file icon + name + size) each with a remove (x).
- On send, attachments attach to the user message: images render as
  thumbnails, files as downloadable chips.
- Use URL.createObjectURL for previews (demo). ponytail: swap for real uploads.

Data model:
Attachment { id, name, size, url, isImage }
Msg { id, role: "user" | "assistant", text, attachments: Attachment[] }
Chat { id, title, messages: Msg[] } // title from first user message

Responses: canned assistant echo so the UI feels live. ponytail: replace with
a Claude API call (claude-opus-4-8 / claude-sonnet-5).
Building blocks: Button, cn, Breadcrumbs, SetPageTitle. Radix icons only
(PlusIcon attach, PaperPlaneIcon send, Pencil1Icon new chat,
FileIcon, Cross2Icon). No chat/upload dependency.

Test scenarios (happy / unhappy):
TC-1 Type + Enter -> user message + assistant reply appended, scrolls down
TC-2 Shift+Enter -> newline, does NOT send
TC-3 Attach an image -> thumbnail chip in composer, then in the sent message
TC-4 Attach a file, remove it -> chip disappears, not sent
TC-5 Send empty with no files -> Send disabled, nothing added
TC-6 New chat -> fresh conversation, prior one kept in history

Done when: centered thread, auto-grow composer, image + file attachments with
preview/remove, New chat history, canned reply, the scenarios above pass,
tokens, light + dark, a11y, lint + types + build pass.