feat(notifications): cross-platform "Pause Notifications" / snooze

Manual DND only existed via the desktop tray (manualDndAtom), so web/mobile
users had no way to pause notifications, and there was no snooze-for-a-duration
anywhere. Add a "Pause Notifications" control in Settings > Notifications:

- Presets: 30 min / 1 hour / 4 hours / Until 8 AM / Until I resume, plus Resume;
  live "Paused until ..." status that flips back on when the snooze lapses.
- Persisted snooze instant (cinny_notification_snooze_until_v1) so it survives a
  reload; 0 = off, SNOOZE_INDEFINITE = until resumed.
- Feeds the existing notification gate (ClientNonUIFeatures, both the message and
  invite monitors) alongside Focus Assist / manual DND / Quiet Hours, suppressing
  notify() and playSound().
- Pure helpers isSnoozeActive/nextTimeAtHour/SNOOZE_INDEFINITE in utils/snooze.ts
  (+5 unit tests); persisted atom in state/notificationSnooze.ts.

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

src/app/utils/snooze.ts

@@ -0,0 +1,21 @@
+// Pure helpers for the cross-platform "pause notifications" / snooze feature.
+// Kept free of jotai/localStorage so they're unit-testable in isolation; the
+// persisted atom lives in state/notificationSnooze.ts.
+
+// Represents "paused until I turn it back on" (a far-future instant).
+export const SNOOZE_INDEFINITE = Number.MAX_SAFE_INTEGER;
+
+// Is the snooze currently active (i.e. suppressing notifications)?
+export function isSnoozeActive(until: number, now: number = Date.now()): boolean {
+  return until > now;
+}
+
+// The soonest future instant at the given local hour (0-23), minute 0 — used for
+// the "Until 8 AM" preset. If that hour has already passed today, rolls to
+// tomorrow.
+export function nextTimeAtHour(hour: number, now: number = Date.now()): number {
+  const d = new Date(now);
+  d.setHours(hour, 0, 0, 0);
+  if (d.getTime() <= now) d.setDate(d.getDate() + 1);
+  return d.getTime();
+}