- Header: required columns (from the row schema's `required` list) get a red
`*` marker so mandatory fields are obvious.
- Save: before the PUT/POST, a client-side required-field check marks the empty
cells and blocks the save with a clear reason; the server's 422 remains the
authority and its messages surface the same way.
- Inline error row: when a row can't be saved (required-missing, 422 validation,
409 duplicate, 403 permission, network, HTTP error), the reason now shows in a
full-width message row directly beneath the offending row — not just a hover
tooltip or the far-off status bar. Cleared on a successful save / reload.
- Editor row-indexing counts data rows only (tr[data-row-id]) so the inserted
error rows never offset cell navigation.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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