feat(auth): restructure oauth2 callback handling and service

frontend/src/app/feature/auth/oauth2/oauth2.service.ts

@@ -0,0 +1,36 @@
+import { inject, Injectable, signal } from '@angular/core';
+import { Router } from '@angular/router';
+import { AuthService } from '@service/auth.service';
+
+
+@Injectable({
+  providedIn: 'root',
+})
+export class Oauth2Service {
+  private router: Router = inject(Router);
+  private authService: AuthService = inject(AuthService);
+  private _error = signal<string>('');
+
+  oauth(provider: string, code: string) {
+    const oauth$ = provider === 'github'
+      ? this.authService.githubAuth(code)
+      : this.authService.googleAuth(code);
+
+    oauth$.subscribe({
+      next: () => {
+        this.router.navigate(['/home']);
+      },
+      error: (err) => {
+        this._error.set(err.error?.message || 'Authentifizierung fehlgeschlagen. Bitte versuchen Sie es erneut.');
+
+        setTimeout(() => {
+          this.router.navigate(['/']);
+        }, 3000);
+      },
+    });
+  }
+
+  public get error() {
+    return this._error;
+  }
+}