Linking photographs to catalogue artworks

Added

• Photo ↔ artwork links: attach library files to a catalogue artwork and navigate both ways — from the work to its files, from a file to the work it documents. This is what finally connects the digital holdings to the catalogue raisonné, instead of keeping them in parallel.
• Certificate issuance directly from the artwork record, without going through another screen.

Fixed

• Focus moved while typing in catalogue forms, losing the field mid-entry. It now stays put.