feat(settings): add the passwordless Sync opt-in page

[vpomerleau]

Because

• The offer to skip the password on later Sync sign-ins has to be made at the one
  point where it costs no extra device prompt: right after a Sync sign-in that used
  a passkey and then a password.

This pull request

• Adds the PasswordlessSyncOptIn page alongside the other post-sign-in interstitials.
• Renders the success banner, offer copy, enable button and decline link.
• Swaps the enable button to a loading label while the wrap is stored, and disables
  the decline link.
• Takes an optional localized error banner for a failed store.
• Adds Storybook stories for the default, loading and error states, plus unit tests.

Issue that this pull request solves

Closes: FXA-14424

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• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

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