Rules after the components layer escape it; pending sign-in uses half the Account page #1001

Two layout follow-ups from the review of #1000.

1. CSS rules outside @layer components. src/ui/app.css closes @layer components { (opened near the top) with an unlabelled brace after the max-height media query. Every rule appended after it is unlayered, and unlayered CSS outranks layered CSS regardless of specificity. For example, the forced-colours rule greying out aria-disabled buttons (inside the layer) is overridden by the normal aria-disabled rule (outside it), so unavailable buttons don't show as GrayText in High Contrast. Move the trailing rules inside the layer and label the closing brace.
2. Pending sign-in on wide windows. While waiting for a code, the Account page has only the band and the code form, so on wide windows the form takes the left half and the right column is empty. Use a single column there.