fix(settings): send a signed-out desktop off the v2 pairing QR page

[dschom]

Because

• After signing out of sync, reloading the v2 QR page on desktop still generated a pairing QR.
• A phone that scanned that QR was approved by a desktop with no account, so its flow fell back to login.

This pull request

• Passes the fxa_status result that AuthAndAccountSetupRoutes already fetches into the ScanQR container.
• Waits for that status and only opens a pairing channel for a verified signed-in user.
• Hard-navigates a signed-out, unverified, disconnected or non-answering browser to /pair.
• Re-asks fxa_status every 5s while the QR is on screen and leaves for /pair, closing the channel, once the browser stops reporting a signed-in user.
• Adds container tests for the pending, signed-out, unverified, disconnected and unanswered cases.

Issue that this pull request solves

Closes: FXA-14657

Checklist

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• 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.

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Screenshots (Optional)

FXA-14657 — signed-out desktop on the v2 QR page

Signed in: QR on screen	After a Sync sign-out: back on /pair

Recording (this branch): a signed-out Firefox opens /pair/authority/scan_qr and is sent to /pair, which starts sign-in; after signing in the QR renders; a Sync sign-out while it is on screen sends the page back to /pair. Locally that lands on the connect page because the sign-out moves Firefox's channel to production; in production /pair gets a signed-out status and starts sign-in, as in the first half.

video.mp4

Other information (Optional)

The ticket asks for a redirect to fxa-root. The redirect goes to /pair rather than a bare / because /pair already owns the signed-out desktop: it begins the sync OAuth flow, lands on fxa-root with the sync params, and returns to the QR page once the browser has an account again.

Firefox sends no browser-to-page message on a sync sign-out, so the QR page polls fxa_status instead. Silence counts as signed out: signOut resets identity.fxaccounts.remote.root, so on a non-production FxA the page loses its web channel entirely.

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