Add pay-for-order component for ticket payment

Introduces PayForOrderComponent with form validation and error handling for ticket payment by order code. Updates routing, navigation, and module declarations to support the new feature.

src/app/navbar/navbar.component.ts

@@ -10,6 +10,7 @@ import { Component, inject, computed, OnInit } from '@angular/core';
 export class NavbarComponent {
   navItems: { label:string, path:string }[] = [
     {label: 'Programm', path: '/schedule'},
+    {label: 'Bezahlen', path: '/checkout/order'},
     {label: 'Film importieren', path: '/admin/movie-importer'},
   ]