Fix date timezone shift in loan detail maturity date computation

Parse date string components directly and use Date.UTC() to avoid
local timezone interpretation that shifts dates by a day in western
timezones when using new Date() with date-only strings.

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

frontend/src/views/loans/LoansView.vue

@@ -367,12 +367,12 @@ async function saveLoanDetail() {
   if (!props.id) return
   saving.value = true
   try {
-    const originDate = new Date(loanForm.value.originationDate)
-    const maturityDate = new Date(originDate)
-    maturityDate.setMonth(maturityDate.getMonth() + loanForm.value.termMonths)
+    const [year, month, day] = loanForm.value.originationDate.split('-').map(Number)
+    const maturityDate = new Date(Date.UTC(year, month - 1 + loanForm.value.termMonths, day))
 
     const payload = {
       ...loanForm.value,
+      originationDate: new Date(Date.UTC(year, month - 1, day)).toISOString(),
       maturityDate: maturityDate.toISOString(),
       escrowAmount: loanForm.value.escrowAmount || undefined,
       extraPayment: loanForm.value.extraPayment || undefined,