feat(apple): keep iOS transfers alive in the background

iOS suspends the process on backgrounding, freezing the core's network
threads so in-flight transfers stall and never fire notifications. Hold a
UIApplication background-task assertion (BackgroundActivityController) while
transfers/shares are active so iOS grants its grace window — long enough to
finish and notify. Released on foreground, on completion, or on expiration.
No UIBackgroundModes added (keeps App Store validation clean); macOS is a
no-op since it already runs unfocused.

Add a localized iOS-only Settings notice explaining the platform limit so it
doesn't read as a bug.

apple/VniDrop/App/AppGraph.swift

@@ -13,6 +13,7 @@ final class AppGraph: ObservableObject {
 	let filePreviewRepository: FilePreviewRepository
 	let approvalCoordinator: ApprovalCoordinator
 	let transferNotificationCoordinator: TransferNotificationCoordinator
+	let backgroundActivity: BackgroundActivityController
 
 	init(dependencies: AppDependencies, coreRepository: CoreRepository? = nil) {
 		self.dependencies = dependencies
@@ -39,6 +40,7 @@ final class AppGraph: ObservableObject {
 			visibility: visibility,
 			messages: messages
 		)
+		self.backgroundActivity = BackgroundActivityController(repository: coreRepository)
 		AppLogger.info("lifecycle", "graph created", ["platform": dependencies.environment.name])
 	}