Configure Sales item search and barcode chooser fields

[sridharkalaibala]

Adds opt-in field preferences under Core Settings → Sale for typed item searches and the fields shown in Sales suggestions and the barcode chooser. Shops without custom preferences retain the current search and display defaults; purchases keep their existing presentation.

Exact barcode scans resolve every matching record and require explicit selection for duplicates. Preferences are validated and scoped to the branch.

Validation: 34 focused search, chooser and MongoDB checks; 220 settings/model checks; ESLint (no errors), formatting, and attribution check.

[github-actions]

Merged to develop. Anyone can test this - you do not need write access.

Try it at https://develop.posnic.io, or run it yourself:

git fetch origin develop && git checkout develop
npm install && npm --prefix api install
npm run dev   # then http://localhost:3000

When you have tested it, say what you did and what happened, and set
QA passed or QA failed. If you cannot set labels, just comment -
a maintainer will.

Reporting that something is broken is as useful as fixing it. It is
better found here than by a shopkeeper.