Implement meeting assistant v1

MeetingAssistant/Recording/UnavailableMeetingAudioSource.cs

@@ -0,0 +1,10 @@
+namespace MeetingAssistant.Recording;
+
+public sealed class UnavailableMeetingAudioSource : IMeetingAudioSource
+{
+    public IAsyncEnumerable<AudioChunk> CaptureAsync(CancellationToken cancellationToken)
+    {
+        throw new PlatformNotSupportedException(
+            "Meeting audio capture is only implemented for the Windows target in this version.");
+    }
+}