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/pages/LoansPage.tsx

@@ -8,7 +8,7 @@ import { Table, TableBody, TableCell, TableHead, TableHeader, TableRow } from "@
 import { formatAmount } from "@/lib/format";
 import { LoanFormDialog } from "@/components/loans/LoanFormDialog";
 import { AmortizationSchedule } from "@/components/loans/AmortizationSchedule";
-import { buildSchedule, currentBalanceFromSchedule } from "@convex-lib/amortization";
+import { buildSchedule } from "@convex-lib/amortization";
 
 export function LoansPage() {
   const loans = useQuery(api.loans.list);
@@ -25,9 +25,9 @@ export function LoansPage() {
         startDate,
         monthlyPayment: loan.monthlyPayment,
         termMonths: loan.termMonths,
+        currentBalance: loan.currentBalance ?? undefined,
       });
-      const balance =
-        loan.currentBalance ?? currentBalanceFromSchedule(schedule.schedule, startDate);
+      const balance = schedule.currentBalance;
       return { loan, schedule, balance };
     });
   }, [loans]);
@@ -47,21 +47,27 @@ export function LoansPage() {
                 <TableHead>Gläubiger</TableHead>
                 <TableHead>Summe</TableHead>
                 <TableHead>Zins</TableHead>
+                <TableHead>Eff. Zins</TableHead>
                 <TableHead>Rate</TableHead>
                 <TableHead>Restschuld</TableHead>
+                <TableHead>Gesamtzinsen</TableHead>
+                <TableHead>Gesamtbetrag</TableHead>
                 <TableHead>Status</TableHead>
                 <TableHead></TableHead>
               </TableRow>
             </TableHeader>
             <TableBody>
-              {enriched.map(({ loan, balance }) => (
+              {enriched.map(({ loan, schedule, balance }) => (
                 <TableRow key={loan._id}>
                   <TableCell>{loan.name}</TableCell>
                   <TableCell>{loan.lender ?? "–"}</TableCell>
                   <TableCell>{formatAmount(loan.principal)}</TableCell>
                   <TableCell>{loan.annualInterestRate.toFixed(2)} %</TableCell>
+                  <TableCell>{loan.effectiveAnnualRate ? `${loan.effectiveAnnualRate.toFixed(2)} %` : "–"}</TableCell>
                   <TableCell>{loan.monthlyPayment ? formatAmount(loan.monthlyPayment) : "–"}</TableCell>
                   <TableCell>{formatAmount(balance)}</TableCell>
+                  <TableCell>{formatAmount(schedule.totalInterest)}</TableCell>
+                  <TableCell>{formatAmount(schedule.totalAmount)}</TableCell>
                   <TableCell>{loan.status}</TableCell>
                   <TableCell className="space-x-1">
                     <Button size="sm" variant="outline" onClick={() => setEditLoan(loan)}>