Use first payment date and maturity date instead of origination date and term

Replace the term months field with a maturity date picker - users know
when their loan ends, not the month count. Compute termMonths from the
two dates. Rename "Origination Date" to "First Payment Date" and adjust
the amortization schedule so payment #1 falls on the entered date.

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

frontend/src/views/loans/LoansView.vue

@@ -84,7 +84,7 @@
               <div>{{ loanDetail.termMonths }} months</div>
             </v-col>
             <v-col cols="6" md="3">
-              <div class="text-medium-emphasis text-caption">Origination Date</div>
+              <div class="text-medium-emphasis text-caption">First Payment Date</div>
               <div>{{ formatDate(loanDetail.originationDate) }}</div>
             </v-col>
             <v-col cols="6" md="3">
@@ -220,9 +220,9 @@
           </v-alert>
           <v-text-field v-model.number="loanForm.originalBalance" label="Original Balance" type="number" prefix="$" />
           <v-text-field v-model.number="loanForm.interestRate" label="Interest Rate (%)" type="number" suffix="%" step="0.125" />
-          <v-text-field v-model.number="loanForm.termMonths" label="Term (months)" type="number" />
           <v-text-field v-model.number="loanForm.monthlyPayment" label="Monthly Payment" type="number" prefix="$" />
-          <v-text-field v-model="loanForm.originationDate" label="Origination Date" type="date" />
+          <v-text-field v-model="loanForm.originationDate" label="First Payment Date" type="date" />
+          <v-text-field v-model="loanForm.maturityDate" label="Maturity Date" type="date" />
           <v-text-field v-model.number="loanForm.escrowAmount" label="Escrow Amount (optional)" type="number" prefix="$" />
           <v-text-field v-model.number="loanForm.extraPayment" label="Extra Payment (optional)" type="number" prefix="$" />
         </v-card-text>
@@ -289,9 +289,9 @@ const payoffForm = ref({
 const loanForm = ref({
   originalBalance: 0,
   interestRate: 0,
-  termMonths: 360,
   monthlyPayment: 0,
   originationDate: '',
+  maturityDate: '',
   escrowAmount: null as number | null,
   extraPayment: null as number | null,
 })
@@ -326,9 +326,9 @@ async function openSetupDialog() {
   loanForm.value = {
     originalBalance: 0,
     interestRate: 0,
-    termMonths: 360,
     monthlyPayment: 0,
     originationDate: new Date().toISOString().split('T')[0],
+    maturityDate: '',
     escrowAmount: null,
     extraPayment: null,
   }
@@ -338,7 +338,6 @@ async function openSetupDialog() {
       const { data } = await loansApi.getLoanDefaults(props.id)
       if (data.originalBalance > 0) loanForm.value.originalBalance = data.originalBalance
       if (data.interestRate > 0) loanForm.value.interestRate = data.interestRate
-      if (data.termMonths > 0) loanForm.value.termMonths = data.termMonths
       prePopulated.value = data.hasSyncData
     } catch {
       // Fall back to zero defaults
@@ -354,9 +353,9 @@ function openEditDialog() {
   loanForm.value = {
     originalBalance: loanDetail.value.originalBalance,
     interestRate: loanDetail.value.interestRate,
-    termMonths: loanDetail.value.termMonths,
     monthlyPayment: loanDetail.value.monthlyPayment,
     originationDate: loanDetail.value.originationDate.split('T')[0],
+    maturityDate: loanDetail.value.maturityDate.split('T')[0],
     escrowAmount: loanDetail.value.escrowAmount,
     extraPayment: loanDetail.value.extraPayment,
   }
@@ -367,13 +366,17 @@ async function saveLoanDetail() {
   if (!props.id) return
   saving.value = true
   try {
-    const [year, month, day] = loanForm.value.originationDate.split('-').map(Number)
-    const maturityDate = new Date(Date.UTC(year, month - 1 + loanForm.value.termMonths, day))
+    const [sY, sM, sD] = loanForm.value.originationDate.split('-').map(Number)
+    const [eY, eM, eD] = loanForm.value.maturityDate.split('-').map(Number)
+    const termMonths = (eY - sY) * 12 + (eM - sM) + 1
 
     const payload = {
-      ...loanForm.value,
-      originationDate: new Date(Date.UTC(year, month - 1, day)).toISOString(),
-      maturityDate: maturityDate.toISOString(),
+      originalBalance: loanForm.value.originalBalance,
+      interestRate: loanForm.value.interestRate,
+      termMonths,
+      monthlyPayment: loanForm.value.monthlyPayment,
+      originationDate: new Date(Date.UTC(sY, sM - 1, sD)).toISOString(),
+      maturityDate: new Date(Date.UTC(eY, eM - 1, eD)).toISOString(),
       escrowAmount: loanForm.value.escrowAmount || undefined,
       extraPayment: loanForm.value.extraPayment || undefined,
     }