Persist countdown inputs and show completion bell in tab title

Co-authored-by: Shelley <shelley@exe.dev>

site/countdown/settings.test.mjs

@@ -0,0 +1,43 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { serializeSettings, parseSettings } from './settings.mjs';
+
+test('settings store only entered minutes and seconds, not countdown state', () => {
+    assert.deepEqual(JSON.parse(serializeSettings('2', '90')), {
+        minutes: '2', seconds: '90',
+    });
+    assert.deepEqual(parseSettings(serializeSettings('2', '90')), {
+        minutes: '2', seconds: '90',
+    });
+});
+
+test('restored seconds have at least two digits and leading zeros are canonicalized', () => {
+    assert.deepEqual(parseSettings(serializeSettings('001', '005')), {
+        minutes: '1', seconds: '05',
+    });
+});
+
+test('empty input is stored as zero', () => {
+    assert.deepEqual(parseSettings(serializeSettings('', '')), {
+        minutes: '0', seconds: '00',
+    });
+});
+
+test('large entered integers round trip without losing precision', () => {
+    const minutes = '123456789012345678901234567890';
+    const seconds = '987654321098765432109876543210';
+    assert.deepEqual(parseSettings(serializeSettings(minutes, seconds)), { minutes, seconds });
+});
+
+test('missing, malformed, or invalid settings restore the default countdown', () => {
+    for (const json of [
+        null, '', 'invalid json', 'null', '[]', '{}', '42',
+        '{"minutes": 1, "seconds": 2}',
+        '{"minutes": "-1", "seconds": "00"}',
+        '{"minutes": "0", "seconds": "1.5"}',
+        '{"minutes": "0", "seconds": ""}',
+        '{"minutes": "0", "seconds": "1e2"}',
+    ]) {
+        assert.deepEqual(parseSettings(json), { minutes: '0', seconds: '00' });
+    }
+});