feat(seo): sync html lang, title, canonical, hreflang and Open Graph tags

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/services/seo.service.spec.ts

@@ -0,0 +1,79 @@
+import { TestBed } from '@angular/core/testing';
+import { provideRouter, Router } from '@angular/router';
+import { Component } from '@angular/core';
+
+import { LanguagePreferenceService } from './language-preference.service';
+import { SeoService } from './seo.service';
+
+@Component({ template: '' })
+class EmptyComponent {}
+
+describe('SeoService', (): void => {
+  const getHref = (selector: string): string | null =>
+    document.head.querySelector(selector)?.getAttribute('href') ?? null;
+
+  beforeEach(async (): Promise<void> => {
+    localStorage.clear();
+    TestBed.configureTestingModule({
+      providers: [
+        provideRouter([
+          { path: '', component: EmptyComponent },
+          { path: 'imprint', component: EmptyComponent },
+        ]),
+      ],
+    });
+    TestBed.inject(SeoService);
+    await TestBed.inject(Router).navigateByUrl('/');
+    TestBed.tick();
+  });
+
+  it('should describe the English start page by default', (): void => {
+    expect(document.documentElement.lang).toBe('en');
+    expect(document.title).toContain('Julian Lechner');
+    expect(getHref('link[rel="canonical"]')).toBe('https://fraujulian.xyz/');
+    expect(getHref('link[hreflang="de"]')).toBe('https://fraujulian.xyz/?lang=de');
+    expect(getHref('link[hreflang="x-default"]')).toBe('https://fraujulian.xyz/');
+  });
+
+  it('should switch language, title and canonical URL together', async (): Promise<void> => {
+    TestBed.inject(LanguagePreferenceService).confirmLanguage('de');
+    await TestBed.inject(Router).navigateByUrl('/imprint');
+    TestBed.tick();
+
+    expect(document.documentElement.lang).toBe('de');
+    expect(document.title).toContain('Impressum');
+    expect(getHref('link[rel="canonical"]')).toBe('https://fraujulian.xyz/imprint?lang=de');
+    expect(document.head.querySelector('meta[property="og:locale"]')?.getAttribute('content')).toBe(
+      'de_AT',
+    );
+  });
+
+  describe('home title', (): void => {
+    beforeEach(async (): Promise<void> => {
+      TestBed.resetTestingModule();
+      vi.useFakeTimers();
+      TestBed.configureTestingModule({
+        providers: [provideRouter([{ path: '', component: EmptyComponent }])],
+      });
+      TestBed.inject(SeoService);
+      await TestBed.inject(Router).navigateByUrl('/');
+      TestBed.tick();
+    });
+
+    afterEach((): void => {
+      vi.useRealTimers();
+    });
+
+    it('should alternate between name and role every 3.5 seconds', (): void => {
+      expect(document.title).toBe('Julian Lechner');
+
+      vi.advanceTimersByTime(3500);
+      TestBed.tick();
+      expect(document.title).toBe('Full Stack Developer');
+
+      vi.advanceTimersByTime(3500);
+      TestBed.tick();
+      expect(document.title).toBe('Julian Lechner');
+    });
+  });
+});