widgetAction derives the form from the request (create without record_id,
update with one) and answers 404 when the field's context hides the
widget on that form, reusing contextAllows as the save path does. An
update-only action can no longer run with a nil record through a direct
POST.
refusedViewModel compared only the top-level type with the controller's
model. It now walks the type through pointers, slices, arrays, maps,
struct fields and the results of exported methods, and the values held
in interface-typed members, refusing the controller's model, any other
GORM model (TableName, a gorm tag, gorm.Model, gorm.DeletedAt) and
html/template's trusted content types anywhere in that structure.
