TASK 22: global audio sync offset (positive delay at the mixer out)

Adds AudioSyncOffsetMs (0..500ms, default 0) that delays the whole
interleaved-stereo mix so audio lands on the video when it runs ahead —
OBS's documented lip-sync fix. Positive-only: advancing audio needs a
video-side delay and is out of the audio layer's scope.

- Services/Audio/AudioSyncDelay.cs: pure delay line, flushed on Configure
- AudioMixer: Func<int> syncOffsetMs seam + delay applied post-limiter
- LayoutStore.Settings + MainViewModel.Audio/VM: load/save + binding
- PreviewPane mic bar: SYNC slider + status dot (IntToSyncBrushConverter)
- AudioSyncDelayTests: identity, negative/beyond-500 clamps, 10ms→960 samples

Reference (external scan): https://obs-versions.com/blog/how-to-fix-audio-delay-on-obs
(audio ahead => positive delay). Verified: build 0 warnings; 3/3 delay tests pass.

Helpers/IntToSyncBrushConverter.cs

@@ -0,0 +1,19 @@
+using System.Globalization;
+using System.Windows.Data;
+using System.Windows.Media;
+
+namespace ytLive.Helpers;
+
+/// <summary>TASK 22: the audio-sync status dot — green when the sync offset is a
+/// no-op (0 ms), amber once the creator has dialled in a delay.</summary>
+public class IntToSyncBrushConverter : IValueConverter
+{
+    private static readonly Brush Ok = new SolidColorBrush(Color.FromRgb(0x34, 0xd3, 0x99));
+    private static readonly Brush Set = new SolidColorBrush(Color.FromRgb(0xf5, 0xb8, 0x4c));
+
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        => value is int i && i == 0 ? Ok : Set;
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}