fix(miniapp): address review feedback

apps/miniapp/src/lib/money.ts

@@ -0,0 +1,24 @@
+export function majorStringToMinor(value: string): bigint {
+  const trimmed = value.trim()
+  const negative = trimmed.startsWith('-')
+  const normalized = negative ? trimmed.slice(1) : trimmed
+  const [whole = '0', fraction = ''] = normalized.split('.')
+  const major = BigInt(whole || '0')
+  const cents = BigInt((fraction.padEnd(2, '0').slice(0, 2) || '00').replace(/\D/g, '') || '0')
+  const minor = major * 100n + cents
+
+  return negative ? -minor : minor
+}
+
+export function minorToMajorString(value: bigint): string {
+  const negative = value < 0n
+  const absolute = negative ? -value : value
+  const whole = absolute / 100n
+  const fraction = String(absolute % 100n).padStart(2, '0')
+
+  return `${negative ? '-' : ''}${whole.toString()}.${fraction}`
+}
+
+export function sumMajorStrings(left: string, right: string): string {
+  return minorToMajorString(majorStringToMinor(left) + majorStringToMinor(right))
+}