Lists: "Add another after saving" batch row entry #55

Part of #49 — Lists parity gaps: GitHub-backed lists, saved views, view modes and collaborative editing

Web behaviour (/help/lists → Entering several rows in a row): an Add another after
saving checkbox next to Add Row keeps the user on the form after each save — the row is saved,
the form empties, a running count is shown, and focus returns to the first field. The checkbox
remembers the choice across lists and across visits. If a save is rejected (e.g. a required
field is empty) nothing is cleared, so the typed values survive.

This is small but it is the difference between entering thirty rows and giving up.

• Add the checkbox to RowEditor, persisted in :core:datastore.
• Keep focus management correct with the software keyboard up.
• Preserve form state on a rejected save.

Tests: repeated-save flow; persistence across screens; values retained on validation error.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.