fix: tolerate invalid device timezones in Week Ahead (Closes #1177)

frontend/today-rollover.js

@@ -4,7 +4,13 @@ function createTodayRollover(options = {}) {
     Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC');
 
   function timeZone() {
-    return options.timeZone?.() || resolvedTimeZone();
+    const candidate = options.timeZone?.() || resolvedTimeZone();
+    try {
+      new Intl.DateTimeFormat('en', { timeZone: candidate }).format(now());
+      return candidate;
+    } catch (_error) {
+      return 'UTC';
+    }
   }
 
   function localDate() {