⚡ Replace File.Create with async FileStream for SettingsManager

[Avicennasis]

💡 What: Replaced the synchronous File.Create(tempPath) call in SettingsManager.SaveSettingsAsync with new FileStream(tempPath, FileMode.Create, FileAccess.Write, FileShare.None, 4096, FileOptions.Asynchronous).

🎯 Why: The previous implementation used File.Create, which performs synchronous I/O at the OS level and blocks thread-pool threads when used with JsonSerializer.SerializeAsync. This caused unnecessary CPU blocking during the file save operation, hindering overall application performance. Using FileOptions.Asynchronous ensures that the subsequent await JsonSerializer.SerializeAsync call utilizes true asynchronous I/O, freeing up threads while the disk write completes.

📊 Measured Improvement:
Note: Due to the WPF target framework requirement (net8.0-windows), BenchmarkDotNet could not execute a baseline measurement successfully within the Linux testing environment, as the necessary runtime framework was unavailable.

Despite the lack of hard metric numbers, replacing blocking I/O with true asynchronous I/O in file writing operations inside an async context is an established architectural optimization in .NET. It prevents thread-pool starvation, reduces CPU context switches, and ensures that the UI/background threads remain highly responsive during serialization and disk write phases.

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PR created automatically by Jules for task 7371063026629044736 started by @Avicennasis

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