feat(fd): show gap-interest renewal preview in Payment dialog before confirming a late renewal

Fetches current balance, gap days since maturity, the linked savings
account's rate, and the resulting gap-interest amount and projected new
balance, and displays them above the dashboard-renewal timing text --
so staff see exactly what a late renewal is about to credit before
confirming anything. Read-only; failure to load the preview doesn't
block the renewal itself, since the existing timing text already
explains the action.

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

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

@@ -174,6 +174,13 @@ export class FixedDepositsService {
     return this.http.post(`/fixeddepositaccounts/${accountId}/credit-gap-interest`, {});
   }
 
+  /** Read-only -- what a Renew is about to do (current balance, linked account's rate, gap days,
+   *  gap-interest amount, projected new balance), shown in the Payment dialog before anything is
+   *  confirmed. Posts nothing; safe to call as often as the dialog needs. */
+  getFdRenewalPreview(accountId: string | number): Observable<any> {
+    return this.http.get(`/fixeddepositaccounts/${accountId}/renewal-preview`);
+  }
+
   /**
    * @param fixedDepositAccount Fixed Deposit Account
    * @returns {Observable<any>}