feat: add Toast component

[brandonmcconnell]

Summary

Adds Toast, an inline message with an optional call to action and a close button.

• variant: info, check, warning, or danger (same names as Callout)
• action: { label, onClick }; pressing it runs onClick and closes the toast
• dismissible (default true): shows the close button; onDismiss runs when it's pressed, and dismissLabel sets its accessible label
• The icon and buttons stay centered on the first line of text when the message wraps. When the message would get narrower than 18rem, the buttons move below it, right-aligned.
• Pass className="rounded-none" to run it edge to edge.

Light	Dark

Test Plan

• pnpm lint and pnpm build
• Checked every story in Storybook in light and dark, including hover states, long text, and a narrow container