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.

Themes/Controls.xaml

@@ -638,6 +638,7 @@
     <BooleanToVisibilityConverter x:Key="BoolToVis"/>
     <Helpers:InverseBoolToVisibilityConverter x:Key="InverseBoolToVis"/>
     <Helpers:NotNullToVisibilityConverter x:Key="NotNullToVis"/>
+    <Helpers:IntToSyncBrushConverter x:Key="IntToSyncBrush"/>
     <Helpers:NullToVisibilityConverter x:Key="NullToVis"/>
     <Helpers:HiddenBoolToVisibilityConverter x:Key="HiddenBoolToVis"/>
     <Helpers:EnumToBoolConverter x:Key="EnumToBool"/>