feat: implement loading state for checkout process and enhance stepper component

src/app/shared/components/stepper/stepper.component.ts

@@ -1,4 +1,10 @@
-import { ChangeDetectionStrategy, Component, contentChildren, signal } from '@angular/core';
+import {
+  ChangeDetectionStrategy,
+  Component,
+  contentChildren,
+  input,
+  linkedSignal,
+} from '@angular/core';
 import { StepComponent } from './step.component';
 import { NgClass } from '@angular/common';
 
@@ -11,7 +17,8 @@ import { NgClass } from '@angular/common';
 })
 export class StepperComponent {
   readonly steps = contentChildren(StepComponent);
-  readonly currentStepIndex = signal(0);
+  readonly initialStepIndex = input(0);
+  readonly currentStepIndex = linkedSignal(() => this.initialStepIndex());
 
   next() {
     const currentSteps = this.steps();