Add realistic loan amortization with currentBalance back-calculation

- Extend schema with effectiveAnnualRate, totalInterest, totalAmount
- Back-calculate paid months from currentBalance and rebuild schedule
- Allow schedule calculation from termMonths without monthlyPayment
- Handle NaN form values gracefully
- Show effective rate, total interest and total amount in UI
- Add amortization unit tests

src/components/loans/AmortizationSchedule.tsx

@@ -27,6 +27,7 @@ export function AmortizationSchedule({
       startDate: new Date(loan.startDate),
       monthlyPayment: loan.monthlyPayment,
       termMonths: loan.termMonths,
+      currentBalance: loan.currentBalance ?? undefined,
     });
   }, [loan]);
 
@@ -71,8 +72,15 @@ export function AmortizationSchedule({
           </TableBody>
         </Table>
         <p className="text-sm text-muted-foreground">
-          Gesamtzinsen: {formatAmount(scheduleResult.totalInterest)} · Enddatum:{" "}
-          {formatDate(scheduleResult.payoffDate.toISOString().slice(0, 10))}
+          Gesamtzinsen: {formatAmount(scheduleResult.totalInterest)} · Gesamtbetrag:{" "}
+          {formatAmount(scheduleResult.totalAmount)} · Enddatum:{" "}
+          {formatDate(
+            Number.isNaN(scheduleResult.payoffDate.getTime())
+              ? undefined
+              : scheduleResult.payoffDate.toISOString().slice(0, 10),
+          )}
+          {loan.effectiveAnnualRate !== undefined &&
+            ` · Effektivzins: ${loan.effectiveAnnualRate.toFixed(2)} %`}
         </p>
       </DialogContent>
     </Dialog>

src/components/loans/LoanFormDialog.tsx

@@ -29,10 +29,13 @@ export function LoanFormDialog({
       lender: "",
       principal: 10000,
       annualInterestRate: 3.5,
+      effectiveAnnualRate: undefined as number | undefined,
       monthlyPayment: undefined as number | undefined,
       termMonths: undefined as number | undefined,
       startDate: new Date().toISOString().slice(0, 10),
       currentBalance: undefined as number | undefined,
+      totalInterest: undefined as number | undefined,
+      totalAmount: undefined as number | undefined,
       status: "aktiv" as "aktiv" | "abbezahlt" | "pausiert",
       notes: "",
     },
@@ -45,10 +48,13 @@ export function LoanFormDialog({
         lender: loan.lender ?? "",
         principal: loan.principal,
         annualInterestRate: loan.annualInterestRate,
+        effectiveAnnualRate: loan.effectiveAnnualRate,
         monthlyPayment: loan.monthlyPayment,
         termMonths: loan.termMonths,
         startDate: loan.startDate,
         currentBalance: loan.currentBalance,
+        totalInterest: loan.totalInterest,
+        totalAmount: loan.totalAmount,
         status: loan.status,
         notes: loan.notes ?? "",
       });
@@ -56,16 +62,30 @@ export function LoanFormDialog({
   }, [loan, form]);
 
   const onSubmit = form.handleSubmit(async (values) => {
+    const monthlyPayment = Number.isNaN(values.monthlyPayment ?? 0) ? undefined : values.monthlyPayment;
+    const termMonths = Number.isNaN(values.termMonths ?? 0) ? undefined : values.termMonths;
+    const currentBalance = Number.isNaN(values.currentBalance ?? 0) ? undefined : values.currentBalance;
+    const effectiveAnnualRate = Number.isNaN(values.effectiveAnnualRate ?? 0) ? undefined : values.effectiveAnnualRate;
+    const totalInterest = Number.isNaN(values.totalInterest ?? 0) ? undefined : values.totalInterest;
+    const totalAmount = Number.isNaN(values.totalAmount ?? 0) ? undefined : values.totalAmount;
+
+    if (monthlyPayment === undefined && termMonths === undefined) {
+      toast.error("Bitte Monatsrate oder Laufzeit angeben");
+      return;
+    }
     try {
       const payload = {
         name: values.name,
         lender: values.lender || undefined,
         principal: values.principal,
         annualInterestRate: values.annualInterestRate,
-        monthlyPayment: values.monthlyPayment,
-        termMonths: values.termMonths,
+        effectiveAnnualRate,
+        monthlyPayment,
+        termMonths,
         startDate: values.startDate,
-        currentBalance: values.currentBalance,
+        currentBalance,
+        totalInterest,
+        totalAmount,
         status: values.status,
         notes: values.notes || undefined,
       };
@@ -118,13 +138,19 @@ export function LoanFormDialog({
             <Label>Jahreszins (%)</Label>
             <Input type="number" step="0.01" {...form.register("annualInterestRate", { valueAsNumber: true })} />
           </div>
+          <div>
+            <Label>Effektiver Jahreszins (%)</Label>
+            <Input type="number" step="0.01" {...form.register("effectiveAnnualRate", { valueAsNumber: true })} />
+          </div>
           <div>
             <Label>Monatsrate</Label>
             <Input type="number" step="0.01" {...form.register("monthlyPayment", { valueAsNumber: true })} />
+            <p className="text-xs text-muted-foreground mt-1">Rate oder Laufzeit angeben</p>
           </div>
           <div>
             <Label>Laufzeit (Monate)</Label>
             <Input type="number" {...form.register("termMonths", { valueAsNumber: true })} />
+            <p className="text-xs text-muted-foreground mt-1">Rate oder Laufzeit angeben</p>
           </div>
           <div>
             <Label>Startdatum</Label>
@@ -134,6 +160,14 @@ export function LoanFormDialog({
             <Label>Aktuelle Restschuld (optional)</Label>
             <Input type="number" step="0.01" {...form.register("currentBalance", { valueAsNumber: true })} />
           </div>
+          <div>
+            <Label>Gesamtzinsen lt. Vertrag (optional)</Label>
+            <Input type="number" step="0.01" {...form.register("totalInterest", { valueAsNumber: true })} />
+          </div>
+          <div>
+            <Label>Gesamtbetrag lt. Vertrag (optional)</Label>
+            <Input type="number" step="0.01" {...form.register("totalAmount", { valueAsNumber: true })} />
+          </div>
           <div className="sm:col-span-2">
             <Label>Notiz</Label>
             <Input {...form.register("notes")} />