fix(apple): restore macOS approval modal and sandboxed file sharing

Approval modal: since the Share/QR sheet auto-opens after creating a transfer,
it is always up when a receiver request arrives, and macOS silently drops a sheet
presented while another is still dismissing — so the approval sheet never appeared.
Drive the approval sheet from explicit state (not a constant binding) and, on
macOS, defer its presentation one dismiss-beat after closing the Share/QR sheet so
the hand-off is serialized. Still a non-dismissable sheet; iOS timing unchanged.

Sandboxed file sharing: the macOS picker released its security scope immediately,
so the core's later import failed with EPERM under the App Store sandbox (the
non-sandboxed .dmg was unaffected). Capture a security-scoped bookmark at pick
time and re-acquire access across shareFiles() — during which the core imports the
bytes — mirroring the receive-folder scoped-access pattern.

apple/VniDrop/Core/FileSystemService.swift

@@ -12,6 +12,10 @@ struct PickedShareFile: Equatable, Identifiable, Sendable {
 	var isTemporaryCopy: Bool = false
 	/// When true, `value` is a directory (path or security-scoped folder URL).
 	var isDirectory: Bool = false
+	/// macOS sandbox: a security-scoped bookmark captured at pick time so access to
+	/// `value` can be re-acquired when the core imports the file (the picker's own
+	/// scope ends immediately). Nil on iOS (which copies into the container instead).
+	var securityScopeBookmark: Data? = nil
 
 	var id: String { value }
 }