fix(pair): center the loading spinner on the supplicant connect screen

[dschom]

Because

• On Android, the loading state shown just before /pair/supplicant/connect_this_device put its spinner in the upper-left corner of the page instead of the center.

This pull request

• Renders the connect_this_device loading state through AppLayout's loading state, so the spinner is centered inside the same page chrome and card as the connect prompt that follows it.
• Adds a container test asserting the spinner renders within the page layout while the pairing channel opens.

Issue that this pull request solves

Closes: FXA-14457

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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Other information (Optional)

• The bug was reported on Android, but the bare spinner had no centering on any platform — the fix is not platform-specific.
• Pair2/Authority/ApproveSignIn/container.tsx has the same bare <LoadingSpinner /> on its own loading state. Left alone here as it is a different screen than the one this ticket covers; worth a follow-up ticket.