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/utils/storage.test.js

@@ -0,0 +1,31 @@
+// @vitest-environment jsdom
+import {afterEach, describe, expect, it, vi} from 'vitest';
+import {storage} from './storage.js';
+
+afterEach(() => {
+    vi.restoreAllMocks();
+    localStorage.clear();
+});
+
+const blockStorage = () => ['getItem', 'setItem', 'removeItem'].forEach((method) =>
+    vi.spyOn(Storage.prototype, method).mockImplementation(() => {
+        throw new DOMException('blocked', 'SecurityError');
+    }));
+
+describe('storage', () => {
+    it('reads, writes and removes a value', () => {
+        storage.set('key', 'value');
+        expect(storage.get('key')).toBe('value');
+
+        storage.remove('key');
+        expect(storage.get('key')).toBeNull();
+    });
+
+    it('does not throw when localStorage throws, and reads null', () => {
+        blockStorage();
+
+        expect(() => storage.set('key', 'value')).not.toThrow();
+        expect(() => storage.remove('key')).not.toThrow();
+        expect(storage.get('key')).toBeNull();
+    });
+});