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/state/notificationSnooze.ts

@@ -0,0 +1,22 @@
+import { atomWithStorage, createJSONStorage } from 'jotai/utils';
+
+/**
+ * Cross-platform "pause notifications" / snooze.
+ *
+ * Unlike `manualDndAtom` (which only mirrors the desktop tray toggle and is
+ * session-only), this is a user-set snooze that works on web/mobile/desktop and
+ * survives a reload. The value is the epoch-ms instant until which notifications
+ * are paused; `0` means not snoozed. `SNOOZE_INDEFINITE` represents "paused until
+ * I turn it back on".
+ *
+ * The pure helpers live in `utils/snooze.ts` (unit-tested); re-exported here so
+ * consumers have a single import site.
+ */
+export { SNOOZE_INDEFINITE, isSnoozeActive, nextTimeAtHour } from '../utils/snooze';
+
+export const notificationSnoozeUntilAtom = atomWithStorage<number>(
+  'cinny_notification_snooze_until_v1',
+  0,
+  createJSONStorage(() => localStorage),
+  { getOnInit: true },
+);