refactor: item 29 cleanup

F16 F17 F18 F19 F20 F21: import polling backs off and pauses in a hidden tab, one date format module, fake timers in the import tests, error codes instead of raw messages, one safe storage wrapper, dead code.

src/context/PreferencesContext.test.jsx

@@ -37,6 +37,21 @@ const answerMe = (settings) => fetchRoutes({
     'PUT /api/users/settings': json(204),
 });
 
+describe('PreferencesProvider storage', () => {
+    it('renders in dark mode when localStorage throws', () => {
+        ['getItem', 'setItem'].forEach((method) => vi.spyOn(Storage.prototype, method).mockImplementation(() => {
+            throw new DOMException('blocked', 'SecurityError');
+        }));
+        try {
+            render(<PreferencesProvider><Spy/></PreferencesProvider>);
+
+            expect(preferences.isDarkMode).toBe(true);
+        } finally {
+            vi.restoreAllMocks();
+        }
+    });
+});
+
 describe('PreferencesProvider language', () => {
     it('takes the language saved in Auth', async () => {
         answerMe({language: 'en', useDarkTheme: true, publicProfile: false});