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".

localization/strings.json

@@ -1757,15 +1757,15 @@
     "notifications_description": {
       "context": "Settings > Notifications: explanation of what notifications are used for.",
       "translations": {
-        "en": "Get notified about new receive requests while VniDrop is in the background.",
-        "fr": "Soyez averti des nouvelles demandes de réception lorsque VniDrop est en arrière-plan.",
-        "es": "Reciba avisos sobre nuevas solicitudes de recepción cuando VniDrop está en segundo plano.",
-        "it": "Ricevi avvisi sulle nuove richieste di ricezione quando VniDrop è in background.",
-        "de": "Werden Sie über neue Empfangsanfragen benachrichtigt, während VniDrop im Hintergrund läuft.",
-        "pt": "Seja notificado sobre novos pedidos de receção enquanto o VniDrop está em segundo plano.",
-        "pl": "Otrzymuj powiadomienia o nowych prośbach o odbiór, gdy VniDrop działa w tle.",
-        "nl": "Ontvang meldingen over nieuwe ontvangstverzoeken terwijl VniDrop op de achtergrond draait.",
-        "ru": "Получайте уведомления о новых запросах на получение, пока VniDrop работает в фоне."
+        "en": "Get notified about transfer activity while VniDrop is in the background.",
+        "fr": "Soyez averti de l’activité des transferts lorsque VniDrop est en arrière-plan.",
+        "es": "Reciba avisos sobre la actividad de las transferencias cuando VniDrop está en segundo plano.",
+        "it": "Ricevi avvisi sull’attività dei trasferimenti quando VniDrop è in background.",
+        "de": "Werden Sie über Übertragungsaktivitäten benachrichtigt, während VniDrop im Hintergrund läuft.",
+        "pt": "Seja notificado sobre a atividade das transferências enquanto o VniDrop está em segundo plano.",
+        "pl": "Otrzymuj powiadomienia o aktywności transferów, gdy VniDrop działa w tle.",
+        "nl": "Ontvang meldingen over overdrachtsactiviteit terwijl VniDrop op de achtergrond draait.",
+        "ru": "Получайте уведомления об активности передач, пока VniDrop работает в фоне."
       }
     },
     "notifications_enabled_message": {
@@ -1810,6 +1810,170 @@
         "ru": "Уведомления отключены для VniDrop. Вы можете включить их в Настройках."
       }
     },
+    "notifications_receive_completed_body": {
+      "context": "Notification body shown when an incoming transfer finishes downloading. {transferName} = transfer name.",
+      "targets": [
+        "apple"
+      ],
+      "args": [
+        {
+          "name": "transferName",
+          "type": "string"
+        }
+      ],
+      "translations": {
+        "en": "“{transferName}” finished downloading.",
+        "fr": "« {transferName} » a fini de se télécharger.",
+        "es": "«{transferName}» terminó de descargarse.",
+        "it": "«{transferName}» è stato scaricato.",
+        "de": "„{transferName}“ wurde vollständig heruntergeladen.",
+        "pt": "«{transferName}» concluiu a transferência.",
+        "pl": "Zakończono pobieranie „{transferName}”.",
+        "nl": "‘{transferName}’ is volledig gedownload.",
+        "ru": "«{transferName}» завершил загрузку."
+      }
+    },
+    "notifications_receive_completed_title": {
+      "context": "Notification title shown when an incoming transfer finishes downloading.",
+      "targets": [
+        "apple"
+      ],
+      "translations": {
+        "en": "Download complete",
+        "fr": "Téléchargement terminé",
+        "es": "Descarga completada",
+        "it": "Download completato",
+        "de": "Download abgeschlossen",
+        "pt": "Transferência concluída",
+        "pl": "Pobieranie zakończone",
+        "nl": "Download voltooid",
+        "ru": "Загрузка завершена"
+      }
+    },
+    "notifications_receive_failed_body": {
+      "context": "Notification body shown when an incoming transfer fails. {transferName} = transfer name.",
+      "targets": [
+        "apple"
+      ],
+      "args": [
+        {
+          "name": "transferName",
+          "type": "string"
+        }
+      ],
+      "translations": {
+        "en": "“{transferName}” couldn’t be received.",
+        "fr": "« {transferName} » n’a pas pu être reçu.",
+        "es": "No se pudo recibir «{transferName}».",
+        "it": "Impossibile ricevere «{transferName}».",
+        "de": "„{transferName}“ konnte nicht empfangen werden.",
+        "pt": "Não foi possível receber «{transferName}».",
+        "pl": "Nie udało się odebrać „{transferName}”.",
+        "nl": "‘{transferName}’ kon niet worden ontvangen.",
+        "ru": "Не удалось получить «{transferName}»."
+      }
+    },
+    "notifications_receive_failed_title": {
+      "context": "Notification title shown when an incoming transfer fails.",
+      "targets": [
+        "apple"
+      ],
+      "translations": {
+        "en": "Download failed",
+        "fr": "Échec du téléchargement",
+        "es": "Error en la descarga",
+        "it": "Download non riuscito",
+        "de": "Download fehlgeschlagen",
+        "pt": "Falha na transferência",
+        "pl": "Pobieranie nie powiodło się",
+        "nl": "Download mislukt",
+        "ru": "Ошибка загрузки"
+      }
+    },
+    "notifications_receiver_completed_body": {
+      "context": "Notification body shown to the sender when a receiver finishes downloading a shared transfer. {receiver} = receiver name, {transferName} = transfer name.",
+      "targets": [
+        "apple"
+      ],
+      "args": [
+        {
+          "name": "receiver",
+          "type": "string"
+        },
+        {
+          "name": "transferName",
+          "type": "string"
+        }
+      ],
+      "translations": {
+        "en": "{receiver} finished receiving “{transferName}”.",
+        "fr": "{receiver} a fini de recevoir « {transferName} ».",
+        "es": "{receiver} terminó de recibir «{transferName}».",
+        "it": "{receiver} ha finito di ricevere «{transferName}».",
+        "de": "{receiver} hat „{transferName}“ vollständig empfangen.",
+        "pt": "{receiver} terminou de receber «{transferName}».",
+        "pl": "{receiver} zakończył odbieranie „{transferName}”.",
+        "nl": "{receiver} heeft ‘{transferName}’ volledig ontvangen.",
+        "ru": "{receiver} завершил получение «{transferName}»."
+      }
+    },
+    "notifications_receiver_completed_title": {
+      "context": "Notification title shown to the sender when a receiver finishes downloading a shared transfer.",
+      "targets": [
+        "apple"
+      ],
+      "translations": {
+        "en": "Transfer received",
+        "fr": "Transfert reçu",
+        "es": "Transferencia recibida",
+        "it": "Trasferimento ricevuto",
+        "de": "Übertragung empfangen",
+        "pt": "Transferência recebida",
+        "pl": "Transfer odebrany",
+        "nl": "Overdracht ontvangen",
+        "ru": "Передача получена"
+      }
+    },
+    "notifications_send_failed_body": {
+      "context": "Notification body shown to the sender when a shared transfer fails. {transferName} = transfer name.",
+      "targets": [
+        "apple"
+      ],
+      "args": [
+        {
+          "name": "transferName",
+          "type": "string"
+        }
+      ],
+      "translations": {
+        "en": "“{transferName}” couldn’t be shared.",
+        "fr": "« {transferName} » n’a pas pu être partagé.",
+        "es": "No se pudo compartir «{transferName}».",
+        "it": "Impossibile condividere «{transferName}».",
+        "de": "„{transferName}“ konnte nicht geteilt werden.",
+        "pt": "Não foi possível partilhar «{transferName}».",
+        "pl": "Nie udało się udostępnić „{transferName}”.",
+        "nl": "‘{transferName}’ kon niet worden gedeeld.",
+        "ru": "Не удалось поделиться «{transferName}»."
+      }
+    },
+    "notifications_send_failed_title": {
+      "context": "Notification title shown to the sender when a shared transfer fails.",
+      "targets": [
+        "apple"
+      ],
+      "translations": {
+        "en": "Sharing failed",
+        "fr": "Échec du partage",
+        "es": "Error al compartir",
+        "it": "Condivisione non riuscita",
+        "de": "Freigabe fehlgeschlagen",
+        "pt": "Falha na partilha",
+        "pl": "Udostępnianie nie powiodło się",
+        "nl": "Delen mislukt",
+        "ru": "Не удалось поделиться"
+      }
+    },
     "notifications_settings_open_failed": {
       "context": "Settings > Notifications: error when the OS notification settings can't be opened.",
       "translations": {