fix(folio): add parseTypedAmount helper matching app's parseCents

Raw user input like "50" must become 5000 cents, not 50. The existing
parseDollarCents helper strips non-digits and so reads "50" as 50 cents,
which is correct for formatted balance text but off by 100x for raw
input from the amount field.

examples/folio/sanderling/predicates.ts

@@ -1,3 +1,19 @@
+// Parses raw user input in the transaction amount field into integer cents.
+// Mirrors the Folio app's parseCents: whole numbers like "50" become 5000
+// cents, decimals like "5.50" become 550, more than 2 decimals or non-numeric
+// input return 0. Leading +/- signs are tolerated and treated as positive.
+export function parseTypedAmount(text: string | undefined | null): number {
+  if (!text) return 0;
+  let trimmed = text.trim().replace(/,/g, "");
+  if (trimmed.startsWith("+") || trimmed.startsWith("-")) trimmed = trimmed.slice(1);
+  if (!/^\d+(\.\d{1,2})?$/.test(trimmed)) return 0;
+  const dot = trimmed.indexOf(".");
+  const whole = dot < 0 ? trimmed : trimmed.slice(0, dot);
+  const frac = dot < 0 ? "" : trimmed.slice(dot + 1);
+  const fracPadded = (frac + "00").slice(0, 2);
+  return parseInt(whole, 10) * 100 + parseInt(fracPadded, 10);
+}
+
 // When the last action is a tap (or double-tap) on the transaction Submit
 // button, the absolute change in total balance must equal the amount the
 // user typed. A double-submit lands two transactions and shifts the balance

pkg/spec/test/folio-typed-amount.test.ts

@@ -0,0 +1,56 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import { parseTypedAmount } from "../../../examples/folio/sanderling/predicates.ts";
+
+test("empty string returns 0", () => {
+  assert.equal(parseTypedAmount(""), 0);
+});
+
+test("undefined returns 0", () => {
+  assert.equal(parseTypedAmount(undefined), 0);
+});
+
+test("null returns 0", () => {
+  assert.equal(parseTypedAmount(null), 0);
+});
+
+test("whole number 50 parses as 5000 cents", () => {
+  assert.equal(parseTypedAmount("50"), 5000);
+});
+
+test("decimal 5.50 parses as 550 cents", () => {
+  assert.equal(parseTypedAmount("5.50"), 550);
+});
+
+test("decimal 5.05 parses as 505 cents", () => {
+  assert.equal(parseTypedAmount("5.05"), 505);
+});
+
+test("one-decimal 0.5 parses as 50 cents", () => {
+  assert.equal(parseTypedAmount("0.5"), 50);
+});
+
+test("non-numeric returns 0", () => {
+  assert.equal(parseTypedAmount("abc"), 0);
+});
+
+test("too many decimals returns 0", () => {
+  assert.equal(parseTypedAmount("5.123"), 0);
+});
+
+test("zero returns 0", () => {
+  assert.equal(parseTypedAmount("0"), 0);
+});
+
+test("leading plus sign tolerated as positive", () => {
+  assert.equal(parseTypedAmount("+50"), 5000);
+});
+
+test("leading minus sign tolerated as positive", () => {
+  assert.equal(parseTypedAmount("-50"), 5000);
+});
+
+test("comma-separated thousands accepted", () => {
+  assert.equal(parseTypedAmount("1,234.56"), 123456);
+});