fix: handle localStorage availability in ThemeService and update tests accordingly

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

@@ -5,6 +5,8 @@ import { ThemeService } from './theme.service';
 describe('ThemeService', () => {
   beforeEach(() => localStorage.clear());
 
+  afterEach(() => vi.unstubAllGlobals());
+
   it('defaults to the operating-system preference', () => {
     vi.stubGlobal(
       'matchMedia',
@@ -34,4 +36,20 @@ describe('ThemeService', () => {
     expect(document.documentElement.dataset['themePreference']).toBe('light');
     expect(listener).toHaveBeenCalledWith('change', expect.any(Function));
   });
+
+  it('works when browser storage is unavailable', () => {
+    vi.stubGlobal('localStorage', undefined);
+    vi.stubGlobal(
+      'matchMedia',
+      vi.fn(() => ({ matches: false, addEventListener: vi.fn() })),
+    );
+
+    const service = TestBed.inject(ThemeService);
+    TestBed.tick();
+
+    expect(service.preference()).toBe('system');
+    service.setPreference('dark');
+    TestBed.tick();
+    expect(document.documentElement.dataset['theme']).toBe('dark');
+  });
 });

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

@@ -8,6 +8,7 @@ const THEME_STORAGE_KEY = 'lmnp-theme';
 @Injectable({ providedIn: 'root' })
 export class ThemeService {
   private readonly document = inject(DOCUMENT);
+  private readonly storage = typeof localStorage === 'undefined' ? null : localStorage;
   private readonly colorSchemeQuery =
     typeof window.matchMedia === 'function'
       ? window.matchMedia('(prefers-color-scheme: dark)')
@@ -26,7 +27,7 @@ export class ThemeService {
       const resolvedTheme =
         preference === 'system' ? (this.systemUsesDark() ? 'dark' : 'light') : preference;
 
-      localStorage.setItem(THEME_STORAGE_KEY, preference);
+      this.storage?.setItem(THEME_STORAGE_KEY, preference);
       this.document.documentElement.dataset['theme'] = resolvedTheme;
       this.document.documentElement.dataset['themePreference'] = preference;
     });
@@ -37,7 +38,7 @@ export class ThemeService {
   }
 
   private readPreference(): ThemePreference {
-    const storedPreference = localStorage.getItem(THEME_STORAGE_KEY);
+    const storedPreference = this.storage?.getItem(THEME_STORAGE_KEY);
     return storedPreference === 'dark' || storedPreference === 'system'
       ? storedPreference
       : 'system';