refactor(apple): adopt Swift 6 language mode

Enable complete strict concurrency and switch the app target to Swift 6.

- Isolate model dependency protocols to @MainActor
- Make the CoreRepository blocking-FFI bridge race-free: nonisolated(unsafe)
  core handle, nonisolated runCore/readSnapshot, @Sendable work block,
  Sendable domain models
- Fix Binding method-reference captures; @preconcurrency imports for
  CoreNFC/AVFoundation/VnidropCore; isolate the NFC/QR delegate helpers

apple/VniDrop/Core/AppPreferences.swift

@@ -2,12 +2,12 @@ import Foundation
 import Combine
 
 /// Receive-destination descriptor, ported from `core/FileSystemService.kt`.
-enum ReceiveFolderKind: String, Codable {
+enum ReceiveFolderKind: String, Codable, Sendable {
 	case fileSystemPath
 	case iosSecurityScopedUrl
 }
 
-struct ReceiveFolder: Equatable, Codable {
+struct ReceiveFolder: Equatable, Codable, Sendable {
 	let kind: ReceiveFolderKind
 	let value: String
 	let displayName: String