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/apple/iOS/VoiceCatiOS/Views/PerUserTuningView.swift

@@ -20,7 +20,7 @@ struct PerUserTuningView: View {
                 Section("Volume") {
                     HStack {
                         Text("Gain")
-                        Slider(value: $gain, in: 0...2, step: 0.05) { _ in
+                        Slider(value: $gain, in: 0...4, step: 0.05) { _ in
                             applyToAllStreams()
                         }
                         .accessibilityLabel("Volume gain for \(user.nickname)")