feat(dock): the LIP-SYNC bay — offset, readout, zero detent

Unit 8. Re-houses the shipped TASK 22 offset in the dock:
- slider −200…+200 ms, snapping to 5 ms ticks (the existing property still clamps and persists)
- readable readout: "0 ms (aligned)" when right, signed otherwise ("+50 ms", "−120 ms")
- presets 0 / +50 / +100; 0 IS the detent — a button is easier to hit than a snap, and "the lip-sync
  is right" deserves one click.

Guard: SyncBayTests pins the binding, the readout, the presets and the snapping, and that the
offset stays the persisted property (no second source of truth).

ViewModels/MainViewModel.Audio.cs

@@ -194,11 +194,21 @@ public partial class MainViewModel
         set
         {
             var clamped = Math.Clamp(value, -500, 500);
-            if (SetProperty(ref _audioSyncOffsetMs, clamped))
-                ScheduleSave();
+            if (!SetProperty(ref _audioSyncOffsetMs, clamped)) return;
+            OnPropertyChanged(nameof(SyncOffsetLabel));
+            ScheduleSave();
         }
     }
 
+    /// <summary>"0 ms (aligned)", "+50 ms", "−120 ms" — what the dock shows beside the slider.</summary>
+    public string SyncOffsetLabel => AudioSyncOffsetMs == 0
+        ? "0 ms (aligned)"
+        : (AudioSyncOffsetMs > 0 ? "+" : "−") + Math.Abs(AudioSyncOffsetMs) + " ms";
+
+    /// <summary>Preset buttons in the LIP-SYNC bay. 0 is the detent: the offset that means "the
+    /// lip-sync is right", so it deserves one click, not a drag.</summary>
+    internal void SetSyncOffset(int milliseconds) => AudioSyncOffsetMs = milliseconds;
+
     private void ToggleMicMute()
     {
         if (MicMuted)