Archive meeting workflow and screenshot changes

MeetingAssistant/Screenshots/UnavailableActiveWindowScreenshotCapture.cs

@@ -0,0 +1,9 @@
+namespace MeetingAssistant.Screenshots;
+
+public sealed class UnavailableActiveWindowScreenshotCapture : IActiveWindowScreenshotCapture
+{
+    public Task<ActiveWindowScreenshot> CaptureAsync(CancellationToken cancellationToken)
+    {
+        throw new PlatformNotSupportedException("Active-window screenshot capture is only available on Windows.");
+    }
+}