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

convex/schema.ts

@@ -81,10 +81,13 @@ export default defineSchema({
     categoryId: v.optional(v.id("categories")),
     principal: v.number(),
     annualInterestRate: v.number(),
+    effectiveAnnualRate: v.optional(v.number()),
     monthlyPayment: v.optional(v.number()),
     termMonths: v.optional(v.number()),
     startDate: v.string(),
     currentBalance: v.optional(v.number()),
+    totalInterest: v.optional(v.number()),
+    totalAmount: v.optional(v.number()),
     status: loanStatus,
     notes: v.optional(v.string()),
   })