## Problem
On the card-less onboarding sign-in (`/welcome`), the **2FA
verification** step renders with a full-viewport-wide submit button.
The 2FA **verify** and **provision** forms hard-code `width: 100%` on
their root `StyledForm`. That was harmless while `/welcome` rendered
inside the `AuthModal` `medium` card, which bounded the width. Since
#22398 removed v1 onboarding, `/welcome` renders card-less under
`BlankLayout`, so nothing bounds those forms and they stretch to the
full viewport.
Other steps are **not** affected, which is why only 2FA looks wrong:
- Sign-in form: root sets `width: ONBOARDING_CONTENT_BLOCK_WIDTH;
max-width: 100%` -> capped at 440.
- SSO selection / workspace-scope: base container (`min-width: 240`, no
width) -> shrink-to-fit.
- **2FA verify / provision: `width: 100%` -> full viewport.**
## Fix
Cap the two 2FA forms at `ONBOARDING_CONTENT_BLOCK_WIDTH` (with
`max-width: 100%`), so they sit in the same block as the sign-in page
instead of forcing full-width.
## Refactor (same PR)
The verify and provision components (both introduced together in #13141)
duplicated their layout shell. Extracted the shared instruction-text and
main-content blocks into `SignInUpTwoFactorAuthenticationStyles.ts`. The
form container stays local to each component since the element differs
(a `div` in provision, a `form` in verification).
## Verification
- Reproduced the flex box-model at 1280px: `width:100%` root -> 1196px
full-width button; `width: 440px` -> 440px centered button.
- lint + format + typecheck pass on the changed files.
Charles Bochet
75d9e0b93a