Because:
- The Mozilla design system replaces the circular third-party auth
buttons with a unified box-style layout.
- Index/Signin/Signup need structural groundwork (composition,
visibility gating, error banner slot) before FXA-13099 wires the
passkey ceremony.
This commit:
- Adds `BoxButton`, a reusable presentational primitive.
- Adds `AlternativeAuthOptions`, composing divider + optional passkey
button + `ThirdPartyAuth` row + inline error banner. Driven by
`showThirdPartyAuth`, `showPasskeySignin`, and `isStandalone` props.
- Adds a `'box'` variant to `ThirdPartyAuth`; refactors
`ButtonPasskeySignin` to wrap `BoxButton`.
- Wires the wrapper into Index, Signin, and Signup. Moves the
`thirdPartyAuth.loginNoPwView` Glean event to Signin and restores it
to a reachable state for the linked-no-password case.
Closes: FXA-13487