add and order product images in the back office

[haribo]

Part of #73. Lands after the storage.

Build

• The product form gains images: choose files, see them, order them, remove one. The first is the default (docs/design/catalog.md § 5).
• The browser resizes above 2400 px on the long side and converts to JPEG, flattening transparency onto white. Below 2400 px the file goes as it is.
• Alternative text is prompted at upload, per image. A product image without it is flagged, since the design asks for that and a storefront without alt text is a storefront nobody blind can shop.
• The catalogue listing shows the thumbnail, which is what the design has said since it was written.

Design first

What the merchant sees while a file uploads, what a refusal says, and how ordering works are not in the design. They go in before the code, and the screens are drawn before that.

Validation

• Unit tests: the resize keeps the ratio, leaves a small image alone, and produces a JPEG.
• A journey adding an image to a product and finding it in the listing, in the three variants.

[haribo]

Frozen until #81 lands.

Reviewing the mockup for this screen turned up the reason: it drew an input by hand instead of rendering TextField, and a table by hand instead of a table component — inside a page whose whole purpose was to show the system. If the mockup does that, the code does it too.

The screen this issue describes needs a grid, a card, a state pill and a field. None of them exist as components today, so building it now would add a fifth surface drawing its own markup rather than composing.

The mockup is kept and will be redone against the components #81 produces, with two corrections already known: reordering by mouse as well as by keyboard, and the alternative text field rendered by the shared field rather than drawn.