fix(deposits): FD Single Rate Calculator sent tenureInMonths=0 for tenures under 30 days

SYMPTOM
FD Single account create screen, Rate Calculator: entering a tenure
under 30 days (e.g. 27) and clicking "Find Rate" produced a Bad
Request - "The parameter `tenureInMonths` must be greater than 0."

ROOT CAUSE
calculateRate() (fixed-deposit-account-interest-rate-chart-step.
component.ts) always converted the Days-unit tenure to months before
calling the backend calculator:
  tenureInMonths = Math.floor(enteredDays / 30)
For any tenure under 30 days this floors to 0, which the backend
correctly rejects (paired backend commit: "FD Single Rate Calculator
crashed for tenures under 30 days").

FIX
- fixed-deposits.service.ts: calculateFixedDepositInterest() -
  tenureInMonths is now optional, added optional tenureInDays.
- fixed-deposit-account-interest-rate-chart-step.component.ts -
  calculateRate(): for a Days-unit tenure >= 30 days, unchanged
  (tenureInMonths = floor(days/30)); for < 30 days, sends tenureInDays
  instead and leaves tenureInMonths undefined. Also fixed the
  API-failure fallback's periodic-compounding math, which divided by
  tenureInMonths directly and would have produced NaN once
  tenureInMonths could legitimately be undefined - it now falls back
  to (enteredDays / 30) when tenureInMonths is absent.

SCOPE
Only the Rate Calculator preview inside the FD Single/Double/Special
account create screen. Does not touch how depositPeriod /
depositPeriodFrequencyId get sent on actual account submission.

IF THIS BREAKS SOMETHING LATER
- The same 400 reappearing for a short FD Single tenure -> confirm the
  Find Rate network call carries tenureInDays and NOT a zero
  tenureInMonths; check calculateRate()'s branch choosing between the
  two.
- A wrong maturity preview number specifically when the backend API
  call fails and the frontend falls back to its own formula -> check
  the `effectiveMonths = tenureInMonths ?? enteredDays / 30` fallback
  line in calculateRate().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/deposits/fixed-deposits/fixed-deposit-account-stepper/fixed-deposit-account-interest-rate-chart-step/fixed-deposit-account-interest-rate-chart-step.component.ts

@@ -334,14 +334,18 @@ export class FixedDepositAccountInterestRateChartStepComponent implements OnChan
     const frequencyId = this.resolveFrequencyId(this.tenurePeriodUnit);
 
     if (this.principalAmount && this.principalAmount > 0) {
-      // Convert tenure to months for backend API
+      // Convert tenure to months for backend API. A Days tenure under 30 days rounds down to 0
-      let tenureInMonths = 0;
+      // months, which the backend rejects -- send it as tenureInDays instead in that case.
+      let tenureInMonths: number | undefined;
+      let tenureInDays: number | undefined;
       if (this.tenurePeriodUnit === 'Years') {
         tenureInMonths = this.tenureValue * 12;
       } else if (this.tenurePeriodUnit === 'Months') {
         tenureInMonths = this.tenureValue;
-      } else {
+      } else if (enteredDays >= 30) {
         tenureInMonths = Math.floor(enteredDays / 30);
+      } else {
+        tenureInDays = enteredDays;
       }
 
       // Map compounding period type id → months (0=Daily, 1=Daily, 4=Monthly, 5=Quarterly, 6=SemiAnnual, 7=Annual)
@@ -395,6 +399,7 @@ export class FixedDepositAccountInterestRateChartStepComponent implements OnChan
           principalAmount: this.principalAmount,
           annualInterestRate: rate,
           tenureInMonths,
+          tenureInDays,
           interestCompoundingPeriodInMonths: compMonths, // 0 = daily, now supported by backend
           interestPostingPeriodInMonths: postMonths,
           submittedOnDate: submittedOnDateStr,
@@ -417,7 +422,8 @@ export class FixedDepositAccountInterestRateChartStepComponent implements OnChan
                 Math.round(this.principalAmount * Math.pow(1 + r / daysInYearVal, enteredDays) * 100) / 100;
             } else {
               const n = 12 / compMonths;
-              const totalCompoundings = Math.floor(tenureInMonths / compMonths);
+              const effectiveMonths = tenureInMonths ?? enteredDays / 30;
+              const totalCompoundings = Math.floor(effectiveMonths / compMonths);
               const rPerCompounding = rate / (n * 100);
               result.maturityAmount =
                 Math.round(this.principalAmount * Math.pow(1 + rPerCompounding, totalCompoundings) * 100) / 100;

src/app/deposits/fixed-deposits/fixed-deposits.service.ts

@@ -133,7 +133,8 @@ export class FixedDepositsService {
   calculateFixedDepositInterest(params: {
     principalAmount: number;
     annualInterestRate: number;
-    tenureInMonths: number;
+    tenureInMonths?: number;
+    tenureInDays?: number; // alternative to tenureInMonths, for sub-month FD Single tenures
     interestCompoundingPeriodInMonths: number;
     interestPostingPeriodInMonths: number;
     submittedOnDate?: string; // yyyy-MM-dd — enables exact calendar calculation