feat(audio): the mic delay gets a slider, and the log states the applied value

Two gaps in what 7f89534 shipped, both caught by the creator's report that sound still
leads the lips:

- The mic delay had presets only (off / 150 / 200), so the value a slower camera needs —
  roughly 175 ms of residual on top of the 175 ms already applied — could not be set at
  all. Now a 0..500 ms slider with off / 175 / 350 presets.
- Nothing recorded which number was in force. The setting lives in the layout DB, so a
  preset click left no trace in the log; "Audio live: started" now names it.

Confirmed meanwhile from the profile: Audio.MicSyncOffsetMs is 175, so the delay is being
applied — the 150 ms of digital silence at the head of the 12:22 track is the delay line
priming, which is what an applied delay looks like. The residual offset is the camera's
pipeline being slower than 175 ms, not a missing delay.

674 pass, 0 warnings.

Services/Audio/AudioMixer.cs

@@ -235,7 +235,9 @@ public sealed class AudioMixer : IDisposable
         _liveCts = cts;
         _pipe = new NamedPipeAudioWriter();
         _pipe.Start(pipeName);
-        _log?.Invoke($"Audio live: started, pipe '{pipeName}'");
+        // Prove in the log which mic delay is actually in force: the value lives in the layout DB and a
+        // preset click is invisible in a recording.
+        _log?.Invoke($"Audio live: started, pipe '{pipeName}', mic delay {_micSyncOffsetMs?.Invoke() ?? 0} ms");
         _ = Task.Run(() => LiveLoopAsync(_pipe, cts.Token));
     }