Add clickable Toast variant
Toasts can now be made clickable by passing `onClick`, in which case the
whole toast renders as a `<button>`. This is mutually exclusive with the
existing `onClose` close button — an interactive element cannot be nested
inside a button — and the constraint is enforced via a discriminated union
on the props.
- Render as a `<button>` in clickable mode, with hover/active/focus-visible
states and native button resets; otherwise render as a `<div>`.
- Apply the Body SM Medium typography via `<Text>` on the content element so
it applies in both the div and button containers (the close button is icon
only and never needed it).
- Add Clickable and ClickableWithIcon stories with screenshots, and default
`onClick`/`onClose` off in the story args so Storybook's `argTypesRegex`
doesn't force every toast into a button / closable state.
