fix(settings): put v2 supplicant pairing screens on a white background

[dschom]

Because

• Figma puts the mobile pairing screens on a white background; Android and
  Storybook render them light grey.
• .card only gains a white background from mobileLandscape up, so on a phone
  the card is transparent and the grey body fills the screen.

This pull request

• Adds an opt-in whiteBackground prop to AppLayout that whitens the page
  wrapper, leaving the default and dark mode untouched.
• Passes it from the six Pair2/Supplicant screens.
• Covers the prop in the AppLayout suite and the page colour in each
  supplicant suite.

Issue that this pull request solves

Closes: FXA-14461

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• 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.

How to review (Optional)

• Key files/areas to focus on: AppLayout/index.tsx; the rest is one prop per screen plus its test.
• Suggested review order: AppLayout first, then any one supplicant screen.
• Risky or complex parts: none — whiteBackground defaults to false, so every other caller is unchanged.

Screenshots (Optional)

Other information (Optional)

• The ticket names two screens with "e.g."; this covers all six supplicant
  screens, since leaving four grey would swap one inconsistency for a worse one.
• The desktop Authority screens keep the grey page — their white cards need it
  for contrast — so this is supplicant-only.