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/App/AppEnvironment.swift

@@ -17,6 +17,7 @@ struct DeviceInfo {
 	let batteryLevel: String?
 }
 
+@MainActor
 protocol DeviceInfoProvider {
 	func load() async -> DeviceInfo
 }