Hide unavailable sales catalogue page controls

[sridharkalaibala]

Sales catalogue paging showed disabled Previous and Next buttons when those pages did not exist. Hide each unavailable direction, retaining navigation on middle pages and the existing single-page behaviour.

Validation: all 4 sales-tile-pages tests pass, covering 2,053 reachable items, first/last-page visibility and remote page loading. Attribution check passed.

[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.