feat(clients): unify volume boost cap to 4x across all clients

Mic input gain (was 3x) and per-user receive gain (was 2x) had
asymmetric boost ceilings. Raise both, plus the desktop aux input
gain (was 3x), to a uniform 4x (400%) on macOS, iOS, and Windows.

The master Output volume slider is unchanged (still 1x). No core
changes needed: the C ABI only clamps negatives, so the ceilings
live entirely in the client UI sliders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Forms/PerUserTuningDialog.cs

@@ -117,11 +117,11 @@ public sealed class PerUserTuningDialog : Form
         var trkGain = new TrackBar
         {
             AccessibleName = $"Gain for {s.Label}",
-            AccessibleDescription = "Volume level for this stream. 100 is normal (1.0×), 200 is double.",
+            AccessibleDescription = "Volume level for this stream. 100 is normal (1.0×); range 0–400 percent.",
             Location = new Point(12, y + 20),
             Size = new Size(260, 45),
             Minimum = 0,
-            Maximum = 200,
+            Maximum = 400,
             Value = ClampToTrack(gain0),
             TickFrequency = 25,
             SmallChange = 5,
@@ -186,7 +186,7 @@ public sealed class PerUserTuningDialog : Form
     private static int ClampToTrack(float gain)
     {
         int v = (int)Math.Round(gain * 100f);
-        return Math.Max(0, Math.Min(200, v));
+        return Math.Max(0, Math.Min(400, v));
     }
 
     private sealed class StreamRow