Convert components to standalone and implement route-based loading

Refactored Angular components to standalone modules, enhancing modularity and reducing dependency on `AppModule`. Updated routes to facilitate lazy loading for improved performance and maintainability. Replaced `platformBrowserDynamic` with `bootstrapApplication` for modern bootstrapping.

frontend/src/app/header/header.component.ts

@@ -1,7 +1,16 @@
 import {Component, HostListener, Input} from '@angular/core';
+import {CommonModule, NgOptimizedImage} from '@angular/common';
+import {ThemeComponent} from '../theme/theme.component';
+import {RouterLink} from '@angular/router';
 
 @Component({
-  standalone: false,
+  standalone: true,
+  imports: [
+    CommonModule,
+    ThemeComponent,
+    RouterLink,
+    NgOptimizedImage
+  ],
   selector: 'app-header',
   templateUrl: './header.component.html',
   styleUrls: ['./header.component.scss']