♻️ refactor(customer): merge continueReward and continue methods into unified flow

Consolidate the separate continueReward() and continue() methods in the customer
details view into a single unified continue() method that handles both reward
selection and regular checkout flows.

Key changes:
- Remove separate continueReward() method
- Move hasReturnUrl() check to end of continue() method
- Share all validation and setup logic between both flows:
  * Customer validation (canAddCustomer, canAddShippingAddress)
  * Destination updates
  * Guest with order checks
  * Customer/buyer/payer/shipping setup
  * Notification channel updates
- Diverge only at navigation step based on hasReturnUrl()
- Simplify template from 3 conditional buttons to 1 unified button with
  conditional content

Benefits:
- Reduced code duplication (~39 lines removed)
- Consistent validation for both flows
- Enhanced reward flow with full business rule checks
- Single point of maintenance
- Cleaner template with reduced conditional complexity

Related to #5262 (Prämienshop-Modus)

apps/isa-app/src/page/customer/customer-search/details-main-view/details-main-view.component.html

@@ -173,41 +173,19 @@
   </div>
 </shared-loader>
 
-@if (hasReturnUrl()) {
-  <button
-    type="button"
-    (click)="continueReward()"
-    class="w-60 text-white text-lg bg-brand rounded-full px-5 py-3 absolute top-[calc(100vh-19.375rem)] left-1/2 -translate-x-1/2 font-bold disabled:bg-inactive-branch"
-    [disabled]="!(hasKundenkarte$ | async)"
-  >
-    <shared-loader [loading]="showLoader$ | async" spinnerSize="32"
-      >Auswählen</shared-loader
-    >
-  </button>
-} @else {
-  @if (shoppingCartHasNoItems$ | async) {
-    <button
-      type="button"
-      (click)="continue()"
-      class="text-white text-lg bg-brand rounded-full px-5 py-3 absolute top-[calc(100vh-19.375rem)] left-1/2 -translate-x-1/2 font-bold disabled:bg-inactive-branch"
-      [disabled]="showLoader$ | async"
-    >
-      <shared-loader [loading]="showLoader$ | async" spinnerSize="32"
-        >Weiter zur Artikelsuche</shared-loader
-      >
-    </button>
-  }
-
-  @if (shoppingCartHasItems$ | async) {
-    <button
-      type="button"
-      (click)="continue()"
-      class="text-white text-lg bg-brand rounded-full px-5 py-3 absolute top-[calc(100vh-19.375rem)] left-1/2 -translate-x-1/2 font-bold disabled:bg-inactive-branch"
-      [disabled]="showLoader$ | async"
-    >
-      <shared-loader [loading]="showLoader$ | async" spinnerSize="32"
-        >Weiter zum Warenkorb</shared-loader
-      >
-    </button>
-  }
-}
+<button
+  type="button"
+  (click)="continue()"
+  [class]="'text-white text-lg bg-brand rounded-full px-5 py-3 absolute top-[calc(100vh-19.375rem)] left-1/2 -translate-x-1/2 font-bold disabled:bg-inactive-branch' + (hasReturnUrl() ? ' w-60' : '')"
+  [disabled]="hasReturnUrl() ? !(hasKundenkarte$ | async) : (showLoader$ | async)"
+>
+  <shared-loader [loading]="showLoader$ | async" spinnerSize="32">
+    @if (hasReturnUrl()) {
+      Auswählen
+    } @else if (shoppingCartHasItems$ | async) {
+      Weiter zum Warenkorb
+    } @else {
+      Weiter zur Artikelsuche
+    }
+  </shared-loader>
+</button>

apps/isa-app/src/page/customer/customer-search/details-main-view/details-main-view.component.ts

@@ -352,32 +352,12 @@ export class CustomerDetailsViewMainComponent
     this._onDestroy$.complete();
   }
 
-  @logAsync
-  // #5262 Für die Auswahl des Kunden im "Prämienshop-Modus" (Getrennt vom regulären Checkout-Prozess)
-  async continueReward() {
-    if (this.hasReturnUrl()) {
-      this._setSelectedCustomerIdInTab();
-
-      // Restore from preserved context (auto-scoped to current tab) and clean up
-      const context = await this._navigationState.restoreAndClearContext<{
-        returnUrl?: string;
-      }>('select-customer');
-
-      if (context?.returnUrl) {
-        await this._router.navigateByUrl(context.returnUrl);
-        return;
-      }
-
-      // No returnUrl found, navigate to default reward page
-      await this._router.navigate(['/', this.processId, 'reward']);
-    }
-  }
-
   @logAsync
   async continue() {
     if (this.isBusy) return;
     this.setIsBusy(true);
 
+    // Shared validation and setup for all flows
     const canAddCustomer = await this._canAddCustomerAsync();
     if (this.shoppingCartHasItems && !canAddCustomer) {
       this.setIsBusy(false);
@@ -420,6 +400,25 @@ export class CustomerDetailsViewMainComponent
 
     this._setShippingAddress();
 
+    // #5262 Check for reward selection flow before navigation
+    if (this.hasReturnUrl()) {
+      // Restore from preserved context (auto-scoped to current tab) and clean up
+      const context = await this._navigationState.restoreAndClearContext<{
+        returnUrl?: string;
+      }>('select-customer');
+
+      if (context?.returnUrl) {
+        await this._router.navigateByUrl(context.returnUrl);
+      } else {
+        // No returnUrl found, navigate to default reward page
+        await this._router.navigate(['/', this.processId, 'reward']);
+      }
+
+      this.setIsBusy(false);
+      return;
+    }
+
+    // Regular checkout navigation
     if (this.shoppingCartHasItems) {
       await this.#rewardSelectionPopUpFlow(this.processId);
     } else {