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".

apple/VniDrop/Features/Notifications/TransferNotificationCoordinator.swift

@@ -0,0 +1,184 @@
+import Combine
+import Foundation
+
+/// A transfer-lifecycle moment worth a local notification.
+enum TransferNotificationKind: Equatable {
+	case sendFailed          // A share you own failed.
+	case receiveCompleted    // An incoming transfer finished downloading.
+	case receiveFailed       // An incoming transfer failed.
+	case receiverCompleted   // A receiver finished downloading your shared transfer.
+}
+
+/// A notification resolved from core state but not yet published. `transferName`
+/// is the raw name (may be nil); the coordinator localizes and applies fallbacks.
+struct PlannedNotification: Equatable {
+	let id: String
+	let kind: TransferNotificationKind
+	let transferName: String?
+	let receiver: String?
+}
+
+/// Pure: transfer-status notifications for this snapshot, excluding already-published
+/// ids. A terminal transfer yields at most one notification, keyed by (kind, id).
+func plannedTransferNotifications(_ transfers: [Transfer], published: Set<String>) -> [PlannedNotification] {
+	transfers.compactMap { transfer in
+		let kind: TransferNotificationKind
+		switch (transfer.direction, transfer.status) {
+		case (.send, .failed): kind = .sendFailed
+		case (.receive, .done): kind = .receiveCompleted
+		case (.receive, .failed): kind = .receiveFailed
+		default: return nil
+		}
+		let id = transferNotificationId(kind, transferId: transfer.transferId)
+		guard !published.contains(id) else { return nil }
+		return PlannedNotification(id: id, kind: kind, transferName: transfer.transferName, receiver: nil)
+	}
+}
+
+/// Pure: one notification per receiver that has finished downloading a shared
+/// transfer, excluding already-published ids.
+func plannedReceiverNotifications(_ requests: [ReceiverRequestModel], published: Set<String>) -> [PlannedNotification] {
+	requests.compactMap { request in
+		guard request.status == .completed else { return nil }
+		let id = "receiver-completed-\(request.id)"
+		guard !published.contains(id) else { return nil }
+		return PlannedNotification(
+			id: id, kind: .receiverCompleted,
+			transferName: request.transferName,
+			receiver: request.receiverName ?? request.receiverDeviceName
+		)
+	}
+}
+
+private func transferNotificationId(_ kind: TransferNotificationKind, transferId: UInt64) -> String {
+	switch kind {
+	case .sendFailed: return "send-failed-\(transferId)"
+	case .receiveCompleted: return "receive-completed-\(transferId)"
+	case .receiveFailed: return "receive-failed-\(transferId)"
+	case .receiverCompleted: return "receiver-completed-\(transferId)"
+	}
+}
+
+/// Fires local notifications for transfer-lifecycle moments (a receive finishing
+/// or failing, a share failing, a receiver completing), so a user who left the
+/// app can see the outcome. Approval prompts are handled by `ApprovalCoordinator`.
+///
+/// Gated on the OS notification permission (and, on iOS, on being backgrounded).
+/// Each moment is terminal, so it is marked seen the first time it is observed and
+/// never re-published. The first state snapshot — which includes existing history
+/// such as past receives — only primes those ids as seen, so only new transitions
+/// notify.
+@MainActor
+final class TransferNotificationCoordinator: ObservableObject {
+	private let repository: CoreGateway
+	private let notifications: LocalNotificationService
+	private let visibility: AppVisibility
+	private let messages: UiMessageController
+
+	private var published = Set<String>()
+	private var primedTransfers = false
+	private var cancellables = Set<AnyCancellable>()
+
+	init(
+		repository: CoreGateway,
+		notifications: LocalNotificationService,
+		visibility: AppVisibility,
+		messages: UiMessageController
+	) {
+		self.repository = repository
+		self.notifications = notifications
+		self.visibility = visibility
+		self.messages = messages
+
+		repository.statePublisher
+			.sink { [weak self] core in
+				guard let self, core.isInitialized else { return }
+				Task { await self.syncTransfers(core.transfers) }
+			}
+			.store(in: &cancellables)
+
+		repository.signals
+			.sink { [weak self] signal in
+				guard let self else { return }
+				switch signal {
+				case .receiverHistoryChanged(let transferId), .transfersChanged(let transferId):
+					Task { await self.syncReceivers(transferId: transferId) }
+				case .approvalChanged:
+					break
+				}
+			}
+			.store(in: &cancellables)
+	}
+
+	/// iOS suppresses notifications while the user is in the app (the convention);
+	/// macOS presents them even when active (also the convention — the app window
+	/// is usually open), relying on the presenter delegate to show the banner.
+	private var canPublish: Bool {
+		guard notifications.permission == .granted else { return false }
+		#if os(iOS)
+		return !visibility.isForeground
+		#else
+		return true
+		#endif
+	}
+
+	private func syncTransfers(_ transfers: [Transfer]) async {
+		let planned = plannedTransferNotifications(transfers, published: published)
+		guard primedTransfers else {
+			// The first snapshot includes existing history (e.g. past receives).
+			// Mark those terminal transfers seen without notifying, so only new
+			// transitions notify.
+			primedTransfers = true
+			for plan in planned { published.insert(plan.id) }
+			return
+		}
+		for plan in planned { await deliver(plan) }
+	}
+
+	private func syncReceivers(transferId: UInt64) async {
+		let result = await repository.receiverRequests(transferId: transferId)
+		switch result {
+		case .success(let requests):
+			for plan in plannedReceiverNotifications(requests, published: published) {
+				await deliver(plan)
+			}
+		case .failure(let error):
+			messages.error(error)
+		}
+	}
+
+	/// Mark seen unconditionally (a terminal moment notifies at most once), then
+	/// publish only when the gate allows.
+	private func deliver(_ plan: PlannedNotification) async {
+		published.insert(plan.id)
+		guard canPublish else { return }
+		let name = plan.transferName ?? String(localized: L10n.Receive.unknownTransfer)
+		let notification: LocalNotification
+		switch plan.kind {
+		case .sendFailed:
+			notification = LocalNotification(
+				id: plan.id,
+				title: String(localized: L10n.Notifications.sendFailedTitle),
+				body: L10n.Notifications.sendFailedBody(transferName: name))
+		case .receiveCompleted:
+			notification = LocalNotification(
+				id: plan.id,
+				title: String(localized: L10n.Notifications.receiveCompletedTitle),
+				body: L10n.Notifications.receiveCompletedBody(transferName: name))
+		case .receiveFailed:
+			notification = LocalNotification(
+				id: plan.id,
+				title: String(localized: L10n.Notifications.receiveFailedTitle),
+				body: L10n.Notifications.receiveFailedBody(transferName: name))
+		case .receiverCompleted:
+			let receiver = plan.receiver ?? String(localized: L10n.Approval.nearbyDevice)
+			notification = LocalNotification(
+				id: plan.id,
+				title: String(localized: L10n.Notifications.receiverCompletedTitle),
+				body: L10n.Notifications.receiverCompletedBody(receiver: receiver, transferName: name))
+		}
+		if case .failure(let error) = await notifications.publish(notification) {
+			messages.error(error)
+		}
+	}
+}