Changesets: staged deletes of gone records count as done, name stale ops

A web changeset with a staged record delete could not be applied after
the record was deleted elsewhere (e.g. with the CLI), and the error only
said "record N does not exist", so it was hard to find the operation to
remove. A delete of a record that is already gone is now a no-op with a
warning. Other operations that no longer apply fail with their number and
summary (op_summary in the API), and the Changes page marks that row and
offers to remove it from the error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/static/assets/style.css

@@ -154,6 +154,8 @@ td.actions-cell { white-space: nowrap; text-align: right; width: 1%; }
 tr.editing td { background: var(--accent-soft); }
 tr.pending-add td { background: var(--add); }
 tr.pending-change td, tr.pending-disable td, tr.pending-enable td { background: var(--warn-soft); }
+tr.op-failed td { background: var(--err-soft); }
+tr.op-failed td:first-child { box-shadow: inset 3px 0 0 var(--err); }
 tr.pending-delete td { background: var(--del); text-decoration: line-through; color: var(--muted); }
 tr.disabled td:not(.actions-cell), tr.disabled td strong { color: var(--muted); }
 /* Disabled records: hatched, so they stand out from live and pending rows. */