Add timezone preference across time displays

src/lib/ui/stores/time-zone.ts

@@ -0,0 +1,26 @@
+import { derived, writable } from 'svelte/store';
+import { createTimeFormatter, DEFAULT_TIME_ZONE, validTimeZone } from '$lib/time-zone.js';
+
+export const TIME_ZONE_STORAGE_KEY = 'electric-cherry:time-zone';
+export const timeZone = writable(DEFAULT_TIME_ZONE);
+export const timeFormat = derived(timeZone, createTimeFormatter);
+export const timeZonePersistenceError = writable(false);
+
+/** Called only on mount so server rendering never shares a user's preference. */
+export function startTimeZonePreference(): () => void {
+    let saved: string | null = null;
+    try { saved = localStorage.getItem(TIME_ZONE_STORAGE_KEY); } catch { /* Use the default. */ }
+    timeZone.set(validTimeZone(saved) ? saved : DEFAULT_TIME_ZONE);
+    const unsubscribe = timeZone.subscribe(value => {
+        try {
+            localStorage.setItem(TIME_ZONE_STORAGE_KEY, value);
+            timeZonePersistenceError.set(false);
+        } catch { timeZonePersistenceError.set(true); }
+    });
+    function onStorage(event: StorageEvent) {
+        if (event.storageArea !== localStorage || (event.key !== TIME_ZONE_STORAGE_KEY && event.key !== null)) return;
+        timeZone.set(validTimeZone(event.newValue) ? event.newValue : DEFAULT_TIME_ZONE);
+    }
+    window.addEventListener('storage', onStorage);
+    return () => { unsubscribe(); window.removeEventListener('storage', onStorage); };
+}