fix: tolerate unavailable browser timezone data

tests/test_tomorrow_plan_frontend.py

@@ -124,6 +124,20 @@ console.log(JSON.stringify({checks,delays:timers.map(timer=>timer.delay),cleared
     assert result["cleared"] == [1, 2]
 
 
+def test_tomorrow_rollover_falls_back_when_the_reported_timezone_is_invalid():
+    result = run_controller("""
+let scheduled=null;
+const planner=createTomorrowPlan({fetchJson:async()=>({}),localDate:()=> '2026-08-20',timeZone:()=> 'Etc/Unknown'});
+const lifecycle=planner.startLifecycle({
+  check:async()=>false,setTimer:(_fn,delay)=>{scheduled=delay;return 1;},clearTimer:()=>{},
+});
+await lifecycle.run();
+console.log(JSON.stringify({scheduled:Number.isFinite(scheduled)&&scheduled>0}));
+""")
+
+    assert result == {"scheduled": True}
+
+
 def test_tomorrow_plan_has_a_touch_safe_mobile_entry_and_reuses_the_ordered_planner():
     index = INDEX.read_text()
     css = CSS.read_text()