test: refactor ThemeService tests to use a mock localStorage implementation

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

@@ -3,7 +3,16 @@ import { vi } from 'vitest';
 import { ThemeService } from './theme.service';
 
 describe('ThemeService', () => {
-  beforeEach(() => localStorage.clear());
+  const storage = new Map<string, string>();
+
+  beforeEach(() => {
+    storage.clear();
+    vi.stubGlobal('localStorage', {
+      clear: () => storage.clear(),
+      getItem: (key: string) => storage.get(key) ?? null,
+      setItem: (key: string, value: string) => storage.set(key, value),
+    });
+  });
 
   afterEach(() => vi.unstubAllGlobals());
 
@@ -25,14 +34,14 @@ describe('ThemeService', () => {
       'matchMedia',
       vi.fn(() => ({ matches: true, addEventListener: listener })),
     );
-    localStorage.setItem('lmnp-theme', 'system');
+    storage.set('lmnp-theme', 'system');
     const service = TestBed.inject(ThemeService);
     TestBed.tick();
 
     expect(document.documentElement.dataset['theme']).toBe('dark');
     service.setPreference('light');
     TestBed.tick();
-    expect(localStorage.getItem('lmnp-theme')).toBe('light');
+    expect(storage.get('lmnp-theme')).toBe('light');
     expect(document.documentElement.dataset['themePreference']).toBe('light');
     expect(listener).toHaveBeenCalledWith('change', expect.any(Function));
   });