feat(mobile): rebuild the auth views on React Native primitives #126

Context

This is the largest single piece of work in the mobile effort, and the one the spike did
not reduce. Roughly 1,500 lines of views and components plus nine CSS modules are DOM
bound and do not port:

Login, PhoneRegistration, EmailRegistration, MagicLinkSent, OtpInput,
PassKeyRegistration, phoneInput, VerifyMagicLink, TermsModal,
AuthFallbackOptions, OAuthProviderButtons, OAuthCallback, DeviceNameModal,
PassKeyLogin.

Every div, input, and CSS module becomes React Native primitives and StyleSheet.
AuthRoutes is react-router-dom and needs an equivalent for whichever native navigation
library the template settles on.

Scope

• Rebuild the auth views on React Native primitives.
• Keep the flow logic in the shared client rather than duplicating it per view, so the
  two bindings do not drift.
• Match the existing flows rather than redesigning them, unless a redesign is decided
  separately. A redesign changes this estimate materially.

Estimate

Three to five engineer weeks, and the critical path for the mobile template.

Relates to #64.