FieldsPage kept the selected field definition in component state, so
reload lost the selection, fields couldn't be linked/shared, and
back/forward didn't navigate selections — inconsistent with
/vocabularies/:id and /objects/:id.
Move selection into the URL: the route becomes /fields/:key?
(optional segment), FieldList selection navigates, cancel/done
navigates back to /fields, and the page derives the selected def from
the already-cached field-defs query. An unknown or stale key (e.g.
after deleting the selected field) falls back to the create form.
Tests: deep link opens the locked edit form, select→cancel round-trips
through the URL, unknown key falls back to create.
Closes#72
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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