Duplicating a slot in the builder fails when the slot has a value in a Number field #512

What happens

On the Build tab, choosing Duplicate on a slot that has a number in a Number field (a "Qty" column, say) adds nothing. The status rail shows "qty" must be a number.

The builder keeps every slot value as a string (toBuildRow → toStringValues in src/components/build-shared/useBuildState.ts). duplicateRow copies those strings, and addRow POSTs them to /api/signups/[id]/slots unchanged. So 12 goes up as "12". Nothing on the server converts it: SlotValuesSchema is z.record(z.string(), z.unknown()), and validateOneValue in src/services/slot-fields.ts requires typeof value === 'number'.

The comment in duplicateRow says "addRow re-serializes for the API", but it doesn't. Its sibling saveRowValues, which cell edits use, does convert number fields with Number(...) and drops empty ones. That's why editing a number works and duplicating doesn't.

An empty Number cell turns into '', which the server reads as missing. So Duplicate only fails when the source slot has a number in it, and then it fails every time.

The same gap is in handleAddSlot (src/components/build-wysiwyg/BuildWysiwyg.tsx), which seeds the group field's value as a string. The builder's group picker only offers date, enum and text fields, so the builder can't reach it today. A group-by set over the API could.

Fix, in outline

• Move the number conversion out of saveRowValues into a small helper (e.g. toTypedValues(fields, values)).
• Call it from saveRowValues and from addRow before the POST, so Duplicate, Add slot and anything else that goes through addRow send typed values.
• Fix the misleading comment in duplicateRow.

Watch for

• The existing duplicateRow tests in useBuildState.test.ts assert the uncoerced body, with text values only. Update them, don't just add alongside.
• Open PR Fetch fields when a settings save names one the Build tab hasn't loaded #508 also touches useBuildState.ts, so whichever lands second has a small merge to do.
• The issue about refused cell edits leaving a row stuck (filed alongside this one) changes editCell and saveRowValues in the same file.

Done when

• A hook test duplicates a slot with a Number field set to 12 and the POST body holds qty: 12 (a number). An empty Number cell is left out of the body.
• Duplicate on such a slot adds the copy in the running app.
• pnpm lint && pnpm typecheck && pnpm knip && pnpm test pass.

Found by the pipeline's bugs scout in the builder (src/components/build-shared, build-wysiwyg), at 82a180f. Checked by a hook test capturing the POST body plus a DB test calling addSlot with { qty: "12" } (refused) and { qty: 12 } (accepted). https://claude.ai/code/session_014HtUS4AnW9zrtSroCVsxmB