fix(ui): clamp FIRE calculator periods for already-met zero-interest goal

The zero-interest (r === 0) branch of calculatePeriodsToRetire returned
a negative number of periods when the FIRE goal was already met
(totalAmount <= P), since the already-met guard only existed on the
r !== 0 branch. This rendered a past/invalid retirement date.

Clamp the zero-interest result with Math.max(0, ...) so it can never go
negative, mirroring the existing guard's behavior on the other branch.

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Fixed a negative number of periods returned by the _FIRE_ calculator when the goal is already met with no interest rate
 - Fixed an issue with the localization of the country names
 
 ## 3.12.0 - 2026-06-17

libs/ui/src/lib/fire-calculator/fire-calculator.service.spec.ts

@@ -32,6 +32,38 @@ describe('FireCalculatorService', () => {
       expect(periodsToRetire).toBe(9);
     });
 
+    it('should return 0 with no interest rate when the goal is already met', async () => {
+      const r = 0;
+      const P = 2000;
+      const totalAmount = 1900;
+      const PMT = 100;
+
+      const periodsToRetire = fireCalculatorService.calculatePeriodsToRetire({
+        P,
+        r,
+        PMT,
+        totalAmount
+      });
+
+      expect(periodsToRetire).toBe(0);
+    });
+
+    it('should return the correct positive amount of periods to retire with no interest rate', async () => {
+      const r = 0;
+      const P = 1000;
+      const totalAmount = 5000;
+      const PMT = 250;
+
+      const periodsToRetire = fireCalculatorService.calculatePeriodsToRetire({
+        P,
+        r,
+        PMT,
+        totalAmount
+      });
+
+      expect(periodsToRetire).toBe(16);
+    });
+
     it('should return the 0 when total amount is 0', async () => {
       const r = 0.05;
       const P = 100000;

libs/ui/src/lib/fire-calculator/fire-calculator.service.ts

@@ -52,7 +52,7 @@ export class FireCalculatorService {
   }) {
     if (r === 0) {
       // No compound interest
-      return (totalAmount - P) / PMT;
+      return Math.max(0, (totalAmount - P) / PMT);
     } else if (totalAmount <= P) {
       return 0;
     }