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/SettingsView.swift

@@ -246,7 +246,7 @@ struct SettingsView: View {
                                 get: { Double(session.voiceState.inputGain) },
                                 set: { session.setInputGain(Float($0)) }
                             ),
-                            in: 0...3, step: 0.05
+                            in: 0...4, step: 0.05
                         )
                         .accessibilityLabel("Microphone volume")
                         .accessibilityValue("\(Int((session.voiceState.inputGain * 100).rounded())) percent")