feat(notifications): device event notifications (snack + Web Notifications)

Surface device connection state changes (configured target online/offline)
and discovery events (new WLED on LAN, new serial port, devices that
disappear) through a configurable per-event channel matrix:
none / snack / OS / both.

- Backend: long-running mDNS browser + 10 s serial poller in
  core/devices/discovery_watcher.py, gated by user pref. Reuses the
  existing device_health_changed event for online/offline transitions.
  New GET/PUT /api/v1/preferences/notifications endpoint with Pydantic v2
  schema (channel matrix + background-discovery flag + grace/debounce).
  13 new tests, full suite still 899 passing.
- Frontend: features/notifications-watcher.ts with startup-grace +
  flap-debounce + bulk-coalesce pipeline. Web Notifications API for the
  OS channel (no platform-specific code, works in PWA shell).
  New "Notifications" tab in Settings with 4 IconSelect rows + bg toggle
  + permission row + test button. en/ru/zh translations.

Defaults: device_offline=both (urgent), online/discovered=snack, lost=none,
background discovery on. Already-configured devices are filtered from
discovery events to avoid double-notifications.

TODO.md

@@ -1,5 +1,65 @@
 # LedGrab TODO
 
+## Device Event Notifications
+
+Notify the user when LED devices come online/go offline (configured targets), and when new
+WLED/serial devices are discovered or disappear from the LAN/USB. Each event class has a
+configurable channel: `none` | `snack` | `os` | `both`. OS channel uses Web Notifications
+(works in any browser tab and in the PWA shell — no platform-specific Python).
+
+Branch: `feat/device-event-notifications`. Default ON.
+
+### Backend
+
+- [x] `core/devices/discovery_watcher.py` — long-running mDNS browser
+      (`AsyncServiceBrowser` kept alive for the process lifetime) + 10 s serial-port
+      poller. Fires `device_discovered`/`device_lost` via `processor_manager.fire_event`,
+      suppresses events for URLs already in `device_store`. Seeded ports do NOT generate
+      startup-time toasts.
+- [x] Wired into `lifespan` (`main.py`). Gated by `notification_preferences.
+      background_discovery_enabled`. Default True. Stops before health monitor stop.
+- [x] `api/schemas/preferences.py` — `NotificationPreferences` Pydantic v2 model with
+      the 4-event channel matrix, `background_discovery_enabled`, `startup_grace_sec`
+      (0..300), `flap_debounce_sec` (0..60).
+- [x] `api/routes/preferences.py` — `GET/PUT /api/v1/preferences/notifications`,
+      persisted under `db.set_setting("notification_preferences", …)`. Corrupt stored
+      values fall back to defaults instead of 500.
+- [x] Reuses existing `device_health_changed` event from `device_health.py` (already
+      fires online/offline transitions on the same event bus).
+- [x] Tests: 7 in `tests/test_preferences_notifications_api.py`, 6 in
+      `tests/test_discovery_watcher.py`. Full pytest suite still 899 passing.
+
+### Frontend
+
+- [x] `js/features/notifications-watcher.ts` — listens to the three `server:*` DOM
+      events. Applies user prefs. Pipeline: startup grace → flap debounce → bulk
+      coalesce (≥3 events / 800 ms collapse to one summary).
+- [x] Web Notification permission requested from the Settings → Notifications panel
+      via a user-gesture button. State chip reflects granted/denied/default.
+- [x] Settings panel — new "Notifications" subtab between Backup and Appearance.
+      4 IconSelects (`none`/`snack`/`os`/`both`) + background-discovery toggle +
+      permission row + Test-notification button.
+- [x] i18n: `settings.notifications.*` and `notifications.*` keys in en/ru/zh.
+
+### Verification
+
+- [x] `npx tsc --noEmit` clean, `npm run build` produces 2.5 MB bundle.
+- [x] `ruff check src/ tests/` clean. 899/899 pytest pass.
+- [x] App import smoke-test (`from ledgrab.main import app`) loads 233 routes
+      without errors.
+- [ ] Real-hardware test pending — verify on user's network:
+      (1) plug a fresh WLED in → snack toast appears, (2) configure it → next
+      offline transition fires both snack + OS toast, (3) Background-discovery
+      toggle off → no more discovered/lost events.
+
+### Out of scope for v1
+
+- Per-device-type granularity (we ship one matrix per event-type, no device-type split)
+- Per-device mute list (deferred — user can globally toggle off if noisy)
+- Native OS toast via Windows winrt API (Web Notifications cover the use case;
+  also avoids the `os_notification_listener` feedback loop)
+- Notification history panel — could land later as the reserved `alerts` dashboard cell
+
 ## Server shutdown action
 
 Let user choose what happens to LED targets on server shutdown.