feat(apple): local notifications for transfer lifecycle events

Adds background notifications for the "the thing you were waiting for is
done" moments, alongside the existing incoming-approval-request one:

  - a receive finished downloading            (receive -> done)
  - a receive failed / was interrupted         (receive -> failed)
  - a share you own failed                      (send -> failed)
  - a receiver finished downloading your share  (receiver status completed)

A new TransferNotificationCoordinator observes core state + signals and
publishes these; the decision of which moments notify is a pure function
(plannedTransferNotifications / plannedReceiverNotifications), unit-tested
independently. The first state snapshot only primes existing history as seen
so launch doesn't spam.

Notification permission is now the single source of truth. The in-app
notifications toggle and its decoupled UserDefaults preference are gone;
the Settings section shows an "Allow notifications" button that requests the
OS permission (or deep-links to Settings once decided), and notifications
gate purely on `permission == .granted`.

macOS delivery fixes:
  - add a UNUserNotificationCenterDelegate so banners present even while the
    app is active (the app window is usually open on macOS)
  - present-when-active on macOS, suppress-when-foregrounded on iOS
  - reserve the notification id before awaiting publish: the CombineLatest
    fired several times and re-added the same identifier, which macOS
    coalesces into a silent update with no banner
  - LocalNotificationService seeds its permission at init so gating can't
    race a not-yet-refreshed .notDetermined

Eight localized title/body strings added (apple-only); the shared
notifications_description copy is generalized from "receive requests" to
"transfer activity".

shared/src/commonMain/composeResources/values-ru/strings.xml

@@ -117,7 +117,7 @@
     <string name="nav_send">Отправить</string>
     <string name="nav_settings">Настройки</string>
     <string name="network_title">Сеть</string>
-    <string name="notifications_description">Получайте уведомления о новых запросах на получение, пока VniDrop работает в фоне.</string>
+    <string name="notifications_description">Получайте уведомления об активности передач, пока VniDrop работает в фоне.</string>
     <string name="notifications_enabled_message">Уведомления включены.</string>
     <string name="notifications_local_title">Разрешить уведомления</string>
     <string name="notifications_permission_denied">Уведомления отключены для VniDrop. Вы можете включить их в Настройках.</string>