Persist countdown inputs and show completion bell in tab title

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

site/countdown/settings.mjs

@@ -0,0 +1,26 @@
+import { integer } from './timer.mjs';
+
+export const SETTINGS_KEY = 'countdown-settings';
+
+export function serializeSettings(minutes, seconds) {
+    return JSON.stringify({
+        minutes: String(integer(minutes)),
+        seconds: String(integer(seconds)),
+    });
+}
+
+export function parseSettings(json) {
+    try {
+        const settings = JSON.parse(json);
+        if (typeof settings?.minutes === 'string' && /^\d+$/.test(settings.minutes) &&
+            typeof settings?.seconds === 'string' && /^\d+$/.test(settings.seconds)) {
+            return {
+                minutes: String(integer(settings.minutes)),
+                seconds: String(integer(settings.seconds)).padStart(2, '0'),
+            };
+        }
+    } catch {
+        // Missing or invalid storage is equivalent to a fresh countdown.
+    }
+    return { minutes: '0', seconds: '00' };
+}