Fix 5 build issues: add ButtonStyles to project, actor isolation, buffer mutability

- Re-ran xcodegen so ButtonStyles.swift is included (fixes PrimaryButtonStyle
  and ScaleButtonStyle not in scope in LoginView and LANTriggerSection)
- BackgroundTaskManager: access BackupEngine.shared inside @MainActor Task
  instead of nonisolated context
- SFTPService: change var buffer → let (never mutated)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Services/SFTPService.swift

@@ -89,7 +89,7 @@ final class SFTPService: NASTransferProtocol {
                 filePath: remotePath,
                 flags: [.write, .create, .truncate]
             ) { file in
-                var buffer = ByteBuffer(data: data)
+                let buffer = ByteBuffer(data: data)
                 try await file.write(buffer, at: 0)
             }
             progress(total, total)