refactor(notifications): quiet-hours uses shared tested time-window helper

isInQuietHours was a hand-rolled, untested duplicate of the overnight-window
logic. Replace it with the shared, unit-tested isWithinTimeWindow (utils/
timeWindow.ts) - identical behavior for valid HH:MM inputs, more robust on
malformed ones (returns false rather than doing NaN math), and now covered by
timeWindow.test.ts. One implementation instead of two.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -15,6 +15,7 @@ import {
 import { focusAssistActiveAtom } from '../../state/focusAssist';
 import { manualDndAtom } from '../../state/manualDnd';
 import { isSnoozeActive, notificationSnoozeUntilAtom } from '../../state/notificationSnooze';
+import { isWithinTimeWindow } from '../../utils/timeWindow';
 import { roomToUnreadAtom } from '../../state/room/roomToUnread';
 import LogoSVG from '../../../../public/res/lotus.png';
 import LogoUnreadSVG from '../../../../public/res/lotus-unread.png';
@@ -69,18 +70,6 @@ import {
 // the async invite-atom population lands first and isn't mistaken for new invites.
 const INVITE_NOTIFY_ARM_DELAY_MS = 3000;
 
-function isInQuietHours(start: string, end: string): boolean {
-  const now = new Date();
-  const [sh, sm] = start.split(':').map(Number);
-  const [eh, em] = end.split(':').map(Number);
-  const cur = now.getHours() * 60 + now.getMinutes();
-  const s = sh! * 60 + (sm ?? 0);
-  const e = eh! * 60 + (em ?? 0);
-  // start===end means zero-length window → treat as disabled (no quiet hours)
-  if (s === e) return false;
-  return s < e ? cur >= s && cur < e : cur >= s || cur < e;
-}
-
 function SystemEmojiFeature() {
   const [twitterEmoji] = useSetting(settingsAtom, 'twitterEmoji');
 
@@ -259,7 +248,7 @@ function InviteNotifications() {
       focusAssistActive ||
       manualDnd ||
       isSnoozeActive(snoozeUntil) ||
-      (quietHoursEnabled && isInQuietHours(quietHoursStart, quietHoursEnd));
+      (quietHoursEnabled && isWithinTimeWindow(quietHoursStart, quietHoursEnd));
     if (quietActive) return;
 
     if (showNotifications && notificationPermission('granted')) {
@@ -539,7 +528,7 @@ function MessageNotifications() {
         focusAssistActive ||
         manualDnd ||
         isSnoozeActive(snoozeUntil) ||
-        (quietHoursEnabled && isInQuietHours(quietHoursStart, quietHoursEnd));
+        (quietHoursEnabled && isWithinTimeWindow(quietHoursStart, quietHoursEnd));
       if (quietActive) return;
 
       if (showNotifications && notificationPermission('granted')) {