feat: update theme preference handling in ThemeService and index.html

frontend/src/app/services/theme.service.spec.ts

@@ -3,9 +3,26 @@ import { vi } from 'vitest';
 import { ThemeService } from './theme.service';
 
 describe('ThemeService', () => {
+  beforeEach(() => localStorage.clear());
+
+  it('defaults to the operating-system preference', () => {
+    vi.stubGlobal(
+      'matchMedia',
+      vi.fn(() => ({ matches: true, addEventListener: vi.fn() })),
+    );
+    const service = TestBed.inject(ThemeService);
+    TestBed.tick();
+
+    expect(service.preference()).toBe('system');
+    expect(document.documentElement.dataset['theme']).toBe('dark');
+  });
+
   it('persists explicit and system preferences', () => {
     const listener = vi.fn();
-    vi.stubGlobal('matchMedia', vi.fn(() => ({ matches: true, addEventListener: listener })));
+    vi.stubGlobal(
+      'matchMedia',
+      vi.fn(() => ({ matches: true, addEventListener: listener })),
+    );
     localStorage.setItem('lmnp-theme', 'system');
     const service = TestBed.inject(ThemeService);
     TestBed.tick();

frontend/src/app/services/theme.service.ts

@@ -40,6 +40,6 @@ export class ThemeService {
     const storedPreference = localStorage.getItem(THEME_STORAGE_KEY);
     return storedPreference === 'dark' || storedPreference === 'system'
       ? storedPreference
-      : 'light';
+      : 'system';
   }
 }