translator: a per-field translate action in the admin

[denis-urban]

Problem

Translation is all-or-nothing per document. An editor who wants to retranslate one paragraph, or fix
one field the model got wrong, has to re-run the whole document — which under overwrite also
discards any hand edits made to other fields in that locale, and under skip_existing cannot touch the
field at all because it already has a value.

The blast radius also matters on failure: a provider error partway through a large document is a
document-sized problem, where it could have been a field-sized one.

Suggested shape

A per-field translate action in the admin: an icon or button next to a localized field that translates
just that field into the current locale. The plumbing largely exists — the field-level config already
knows which fields are translatable (isTranslatableLeaf, the exclude flag), and
createOpenAIProvider returns a provider whose translate(Record<number, string>, sourceLng, targetLng) is directly callable with a single entry.

Note on scope

This would not reach fields the field walker cannot see. In our case that is uiStrings, a localized
json field on a Payload global, which no amount of per-field UI helps with since the plugin has
no globals key at all. We are routing around that separately and are deliberately not asking for
globals support.