fix(slack): don't let one tap flatten a hand-made split

Splits on orders are made by hand, so a third participant or an uneven
share is a deliberate decision — and "Make it just me" deleted every
split row regardless. A one-tap button silently destroying an
arrangement made with more care than the tap that undid it is the same
failure shape as the rewrite that dropped `settled`.

Now it refuses when a participant other than the two consumers is
present, or when the share is not 50. Verified against the running stack:
a three-way split and a 70/30 both survive a press; a plain 50/50 still
toggles off and back on.

Also: the nudge reads share state instead of assuming false. Today a
freshly ingested order has no splits — the 140 that do were split by hand
after the backfill, not by a rule — but the label drives a destructive
button, so a wrong assumption there costs data rather than a cosmetic
error. One query is cheaper.

src/app/api/slack/interactive/route.ts

@@ -4,6 +4,7 @@ import { verifySlackSignature, participantForSlackUser } from "@/lib/slack-verif
 import { nudgeBlocks, detailsModal } from "@/lib/slack-blocks";
 import {
   RATINGS,
+  OWNER_PARTICIPANT_ID,
   SECOND_CONSUMER_ID,
   merchantVerdict,
   type Rating,
@@ -200,10 +201,23 @@ async function handleModalSubmit(payload: {
  * 2026-01-09 — so there is no settled historical obligation to lose.
  */
 async function toggleShare(transactionId: number) {
-  const existing = await queryRaw<{ participant_id: number }>(
-    `SELECT participant_id FROM transaction_splits WHERE transaction_id = $1`,
+  const existing = await queryRaw<{ participant_id: number; share_percent: string }>(
+    `SELECT participant_id, share_percent FROM transaction_splits WHERE transaction_id = $1`,
     [transactionId]
   );
+
+  // Refuse to touch an arrangement this button cannot express. Splits are made
+  // by hand here, so a third participant or an uneven share is deliberate — and
+  // a one-tap button that silently flattened it would destroy a decision made
+  // with more care than the tap that undid it.
+  const foreign = existing.filter(
+    (e) => e.participant_id !== SECOND_CONSUMER_ID && e.participant_id !== OWNER_PARTICIPANT_ID
+  );
+  const uneven = existing.some(
+    (e) => e.participant_id === SECOND_CONSUMER_ID && Number(e.share_percent) !== 50
+  );
+  if (foreign.length || uneven) return;
+
   if (existing.some((e) => e.participant_id === SECOND_CONSUMER_ID)) {
     await queryRaw(`DELETE FROM transaction_splits WHERE transaction_id = $1`, [
       transactionId,