Adds a user preference to remove the `max-w-3xl` constraint on agent
chat messages, letting the content fill the available viewport width.
The toggle lives in **Agents > Settings > Behavior** under "Chat Layout"
and persists via `localStorage` (`agents.chat-full-width`). A
`useSyncExternalStore` hook (`useChatFullWidth`) provides same-tab
reactivity so flipping the toggle updates all mounted consumers
immediately — the chat timeline, chat input, the Suspense skeleton, and
the in-page loading view.
This was requested by a customer and is an individual user setting, so
it seems fine to add.