forms(qml): no way to load an existing payload into a DynamicForm (JSON -> draft), which a collection slot needs to edit stored rows

[Yaraslaut]

Verification: inferred from reading DynamicForm.qml, CollectionView.qml and WizardView.qml on master 05222af.

Every prefill path writes control text: CollectionView locates field_<name> and sets .text; WizardView calls setFieldValue(name, text). Nothing converts a wire value into the text the form's draft holds, so a caller editing an existing record has to do it by hand per kind — {num,den,dp} → locale digits for a Quantity, an ISO string → display-zone wall clock for a Timestamp, a value → its option's valueJson for a closed set.

This bites hardest for a std::vector<Row> drawn by a slot (#809): the slot's rows are {member: cellText}, and turning a stored [{"sieve":31.5,"passing":{"num":1000,"den":10,"dp":1}}] back into cell texts is the inverse of encodeFieldText, which does not exist.

Proposal: DynamicForm.decodeFieldValue(field, jsonValue) -> text (the inverse of encodeFieldText, locale- and zone-aware) and DynamicForm.prefillFromJson(payloadText) built on it, covering object-array rows cell by cell.

Closes when form.prefillFromJson(form.previewLine) on a ready form reproduces the same previewLine, for every field kind including a slot-drawn collection.