feat(slack): ask the other person too, and put rating before sharing

Two halves of the same requirement, one of which was quietly missing.

Sharing split the money but never reached her: she is not in #smarthome,
so the card whose caption said "both verdicts welcome" was one she could
not see. Now a share DMs her a card of her own. A DM rather than adding
her to the channel, so her surface stays "orders that concern me"
instead of the whole house's ops feed. She was already in SLACK_USER_MAP,
so her press files under participant 4.

Only on the press that turns sharing ON, and only when someone else did
the sharing. Re-notifying on every later rating press would turn one
shared meal into a stream of DMs, which is how a nudge gets muted.

Her card carries no share button: she is being told it was shared, not
asked to decide, and two people toggling one split from separate copies
of a card is a race with no upside.

The app still holds no Slack bot token — it returns a notify instruction
and n8n sends it, the same shape as the modal open. If SLACK_USER_MAP has
no id for her the DM is skipped silently: the split is correct and
complete either way, and failing the press over an unaddressable nudge
would be the worse trade.

Card reordered to rate -> details -> share. You judge the food, then
decide who pays for it; asking "was this shared?" first inverts the
order a person thinks in. The status caption moved under the share button
it describes rather than sitting orphaned mid-card.

src/lib/slack-verify.ts

@@ -47,6 +47,16 @@ export function verifySlackSignature(
  * two-person household a wrong attribution is not a rounding error, it is the
  * other person's opinion recorded under your name.
  */
+/** The reverse: which Slack user is this participant, for DMing them. */
+export function slackUserForParticipant(participantId: number): string | null {
+  const map = process.env.SLACK_USER_MAP ?? "";
+  for (const pair of map.split(",")) {
+    const [slack, participant] = pair.split(":").map((s) => s.trim());
+    if (slack && Number(participant) === participantId) return slack;
+  }
+  return null;
+}
+
 export function participantForSlackUser(slackUserId: string): number | null {
   const map = process.env.SLACK_USER_MAP ?? "";
   for (const pair of map.split(",")) {