fix(settings): Prevent pairing when the mobile browser is signed in

[dschom]

Because

• Pairing signs a device in to Sync. A mobile browser that already holds a verified account has nothing to pair, yet the v2 flow completed with no warning (FXA-14479).

This pull request

• Routes a signed-in supplicant from /pair to the pairing dead-end screen before the channel opens, so the Connect card is never shown.
• Adds a signed_in reason to TimeoutAndCancel with new Fluent copy telling the user to sign out and retry, plus a Storybook story.
• Lists the new reason in the Glean dtm_mobile.timeout_view extra description.
• Covers the gate (verified user stops; unverified or token-less user still hands off) and the new variant in the page tests.

Issue that this pull request solves

Closes: FXA-14479

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: pages/Pair/Index/index.tsx (the gate), pages/Pair2/Supplicant/TimeoutAndCancel/index.tsx (copy)
• Suggested review order: gate, then the dead-end page and its en.ftl, then tests
• Risky or complex parts: none; the channel is never opened on the new path

Screenshots (Optional)

FXA-14479 — mobile dead-end screen, signed_in reason (Storybook, this branch)

After (this branch)

Other information (Optional)

• This stops a signed-in device on purpose. The account switch QA observed on train 346.3 no longer happens; the user is told to sign out first.
• Desktop is not notified on this path. It waits out the channel and shows its timeout screen.
• Copy is a first draft and needs a UX pass before l10n export.

[vbudhram]

@dschom Works on emulator 👍🏽

combined.mp4