Label printing gets a real print pipeline

Shipped

• CSS print pipeline for label sheets. The screen preview and the printed page now share the same DOM instead of being generated separately — what you see in the live preview is what comes out of the printer, margin for margin.
• An alignment test page. Print one test sheet to confirm your printer's own margins before committing a full sheet of real labels to it.
• The scan loop, closed. Scanning a printed label's QR code now reliably opens the item it's attached to — the label queue, print flow, and the scanner were tested end-to-end together, not just each piece on its own.

See Labels & printing for how the whole pipeline works today, template sizes included.