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/Core/LocalNotificationService.swift

@@ -16,12 +16,32 @@ struct LocalNotification {
 	let body: String
 }
 
+/// Presents notifications even while the app is active. Without a delegate the
+/// system drops the banner when the app is frontmost — very visible on macOS,
+/// where the app window is usually open when a transfer completes.
+private final class NotificationPresenter: NSObject, UNUserNotificationCenterDelegate {
+	func userNotificationCenter(
+		_ center: UNUserNotificationCenter,
+		willPresent notification: UNNotification
+	) async -> UNNotificationPresentationOptions {
+		[.banner, .sound, .list]
+	}
+}
+
 /// Local notification service backed by `UNUserNotificationCenter`.
 @MainActor
 final class LocalNotificationService: ObservableObject {
 	@Published private(set) var permission: NotificationPermission = .notDetermined
 
 	private let center = UNUserNotificationCenter.current()
+	private let presenter = NotificationPresenter()
+
+	init() {
+		center.delegate = presenter
+		// Seed the permission immediately so gating (approval/lifecycle
+		// notifications) never races a not-yet-refreshed `.notDetermined`.
+		Task { _ = await refreshPermission() }
+	}
 
 	func refreshPermission() async -> NotificationPermission {
 		let settings = await center.notificationSettings()
@@ -75,11 +95,6 @@ final class LocalNotificationService: ObservableObject {
 		center.removeDeliveredNotifications(withIdentifiers: [id])
 	}
 
-	func cancelAll() {
-		center.removeAllPendingNotificationRequests()
-		center.removeAllDeliveredNotifications()
-	}
-
 	private static func map(_ status: UNAuthorizationStatus) -> NotificationPermission {
 		switch status {
 		case .authorized, .provisional, .ephemeral: return .granted