feat: enhance checkout process with purchase editing and status handling

- Implemented purchase editing functionality in CheckoutPageComponent, allowing users to modify items in their purchase.
- Added a new guard (checkoutPendingPurchaseGuard) to prevent navigation away from the checkout page while a purchase is in progress.
- Updated the login page to handle return URLs after authentication.
- Enhanced product detail page to support direct purchases with a new buyNow method.
- Introduced new UI elements and logic to handle purchase status, including expired and rejected states in PurchaseStatusPageComponent.
- Improved cart component to allow editing of item quantities with a toggle button.
- Added quantity selector enhancements to disable controls when necessary.
- Updated tests to cover new functionalities and ensure proper behavior of components.

src/app/app.routes.spec.ts

@@ -235,7 +235,7 @@ describe('app routes', () => {
   it('redirects unauthenticated users from /checkout to /login', async () => {
     const { router } = await renderAppAt('/checkout', createTenantServiceStub(), createAuthServiceStub(false));
 
-    expect(router.url).toBe('/login');
+    expect(router.url).toBe('/login?returnUrl=%2Fcheckout');
   });
 
   it('allows authenticated users to access /checkout', async () => {