feat(web-admin): add last-period due amount calc helpers

Pure functions for fq auto-fill of the final installment amount
with override/lock gates, covered by unit tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

web-admin/src/utils/protocol-sample-payment.ts

@@ -409,3 +409,34 @@ export function applyExpandRemainingToInstallment<T extends Record<string, any>>
     })
   }
 }
+
+/** 分期：最后一期应缴 = 合同 − 前 N−1 期；无法计算返回 null；负值钳为 0 */
+export function calcLastPeriodDueAmount(
+  contractAmount: number | null | undefined,
+  priorAmounts: Array<number | string | null | undefined>
+): number | null {
+  // Number(null) === 0；须先排除空值，否则会误算尾期
+  if (contractAmount == null) return null
+  const contract = Number(contractAmount)
+  if (!Number.isFinite(contract)) return null
+  if (!priorAmounts.length) return null
+  let anyFilled = false
+  let sumCents = 0
+  for (const raw of priorAmounts) {
+    if (raw === null || raw === undefined || raw === '') continue
+    const n = Number(raw)
+    if (!Number.isFinite(n)) continue
+    anyFilled = true
+    sumCents += Math.round(n * 100)
+  }
+  if (!anyFilled) return null
+  const last = Math.round(contract * 100 - sumCents) / 100
+  return last < 0 ? 0 : last
+}
+
+export function shouldAutoFillLastPeriodDue(opts: {
+  lastManualOverride: boolean
+  lastLocked: boolean
+}): boolean {
+  return !opts.lastManualOverride && !opts.lastLocked
+}