fix(loan): 원리금균등 PMT 자동계산 — 납입금액 수동입력 불필요

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/views/account/AccountView.vue

@@ -161,7 +161,7 @@ function monthlyInterestOf(loan) {
   return Math.round(Math.abs(loan.balance || 0) * (Number(loan.loanRate) / 100 / 12))
 }
 
-// 상환방식별 자동계산: 원금균등·만기일시는 납입금액 입력 불필요
+// 상환방식별 자동계산: 원금균등·만기일시·원리금균등(PMT) 모두 자동, 나머지만 수동
 const loanAutoResult = computed(() => {
   const loan = repayTargetLoanWallet.value
   if (!loan) return null
@@ -173,6 +173,14 @@ const loanAutoResult = computed(() => {
     const principal = Math.round(Number(loan.loanAmount) / loan.loanMonths)
     return { interest, principal, total: principal + interest, needsInput: false }
   }
+  if (loan.loanMethod === 'EQUAL_PAYMENT' && loan.loanAmount && loan.loanMonths && loan.loanRate) {
+    // PMT 공식: M = P × r(1+r)^n / ((1+r)^n − 1)
+    const r = Number(loan.loanRate) / 100 / 12
+    const n = loan.loanMonths
+    const M = Math.round(Number(loan.loanAmount) * r * Math.pow(1 + r, n) / (Math.pow(1 + r, n) - 1))
+    const principal = Math.max(0, M - interest)
+    return { interest, principal, total: M, needsInput: false }
+  }
   return { interest: null, principal: null, total: null, needsInput: true }
 })