feat(slack): per-item verdicts and a note, in a Slack modal

The card can rate an order but cannot ask which dish or why: a message
cannot collect free text, and an actions block caps at 25 elements while
item counts vary per receipt. A modal is the only Slack-native answer,
and it stays inside Slack — no browser, no app, which is the whole reason
it exists rather than a link.

The overall rating deliberately stays on the card. That is the thing done
every time and it should cost one tap; this is for when something was
notably good or bad.

finance-app holds no Slack bot token by design, so it returns the view
and n8n — which already has the credential — calls views.open. One copy
of the token, no new secret, no compose change.

Item text travels in private_metadata because a submission returns block
ids and values, never labels, so there is otherwise no way back to which
dish a radio button referred to. Capped at 20 rows: a grocery order runs
long and nobody scrolls a modal to rate a tin of tomatoes.

The modal does NOT write the rating. A form that silently reset a
decision the user did not revisit is the same class of bug as the split
rewrite that dropped `settled`.

src/lib/slack-blocks.ts

@@ -100,5 +100,123 @@ export function nudgeBlocks(s: NudgeState) {
         },
       ],
     },
+    {
+      type: "actions",
+      block_id: "details",
+      elements: [
+        {
+          type: "button",
+          action_id: "open_details",
+          text: { type: "plain_text", text: "Add details" },
+          value: `${s.transactionId}:details`,
+        },
+      ],
+    },
   ];
 }
+
+/**
+ * The per-item verdicts and the free-text note, as a Slack modal.
+ *
+ * A message cannot collect free text and an actions block caps at 25 elements,
+ * so this is the only Slack-native way to ask "which dish, and why". It stays
+ * inside Slack — no browser, no app — which is the entire reason it exists
+ * rather than a link.
+ *
+ * The overall rating deliberately stays on the card: it is the thing you do
+ * every time and it should cost one tap. This is for the times something was
+ * notably good or bad.
+ *
+ * `private_metadata` carries the ids because a view_submission arrives as a
+ * fresh request with no reference to the message it came from.
+ */
+export function detailsModal(
+  transactionId: number,
+  participantId: number,
+  merchant: string,
+  items: string[],
+  existing: { note?: string | null; itemVerdicts?: { item: string; verdict: string }[] }
+) {
+  const verdictOf = (item: string) =>
+    existing.itemVerdicts?.find(
+      (v) => v.item.trim().toLowerCase() === item.trim().toLowerCase()
+    )?.verdict ?? null;
+
+  const opt = (text: string, value: string) => ({
+    text: { type: "plain_text", text },
+    value,
+  });
+
+  const itemBlocks = items
+    // Slack allows 100 blocks per view; a grocery order can be long, and past
+    // ~20 rows nobody is scrolling a modal to rate a tin of tomatoes anyway.
+    .slice(0, 20)
+    .map((item, i) => {
+      const current = verdictOf(item);
+      const options = [opt("👍 Great", "loved"), opt("👎 Never again", "never")];
+      return {
+        type: "input",
+        block_id: `item_${i}`,
+        optional: true,
+        // The label carries the item name; Slack truncates at 150 chars.
+        label: { type: "plain_text", text: item.slice(0, 150) },
+        element: {
+          type: "radio_buttons",
+          action_id: "verdict",
+          options,
+          ...(current
+            ? { initial_option: options.find((o) => o.value === current) }
+            : {}),
+        },
+      };
+    });
+
+  return {
+    type: "modal",
+    callback_id: "order_details",
+    private_metadata: JSON.stringify({
+      t: transactionId,
+      p: participantId,
+      // The item text is not recoverable from the submission — Slack returns
+      // block ids and values, not labels — so it travels with the view.
+      i: items.slice(0, 20),
+    }),
+    title: { type: "plain_text", text: "Order details" },
+    submit: { type: "plain_text", text: "Save" },
+    close: { type: "plain_text", text: "Cancel" },
+    blocks: [
+      { type: "section", text: { type: "mrkdwn", text: `*${merchant}*` } },
+      {
+        type: "input",
+        block_id: "note",
+        optional: true,
+        label: { type: "plain_text", text: "Anything worth remembering?" },
+        element: {
+          type: "plain_text_input",
+          action_id: "value",
+          multiline: true,
+          initial_value: existing.note ?? undefined,
+          placeholder: {
+            type: "plain_text",
+            text: "e.g. the biryani was decent, sides were cold",
+          },
+        },
+      },
+      ...(itemBlocks.length
+        ? [{ type: "divider" }, ...itemBlocks]
+        : [
+            {
+              type: "context",
+              elements: [
+                {
+                  type: "mrkdwn",
+                  // Uber itemises groceries but not restaurant orders, so an
+                  // empty list is the receipt, not a failure.
+                  text: "_This receipt has no itemised list._",
+                },
+              ],
+            },
+          ]),
+    ],
+  };
+}