feat(clients): persist input settings, add mic input gain, fix iOS chat + VoiceOver

Input mode (VAD/PTT/Always-On), VAD threshold, and the new mic gain were
applied to the core + UI but never saved, so every relaunch reset to VAD
defaults. Each client now persists them and re-applies on connect:
  - iOS: UserDefaults (SessionState.loadAndApplyVoiceSettings + setter writes)
  - macOS: UserDefaults via MainWindowController didSet + loadPersistedAudioSettings
    (settings window also restores the VAD slider from the stored threshold)
  - Windows: new Models/VoiceSettings.cs (JSON at %AppData%\VoiceCat\voice.json,
    mirrors FeedbackSettings) loaded/applied in MainForm

Add global send-side mic gain API vc_set_input_gain (applied to MIC PCM in
on_capture_frame before the VAD gate, clamped to int16) + Swift/C# bindings,
and a 0-300% (default 100%) mic-volume slider on all three clients.

Fix iOS chat: ChatView called sendText(scope:.channel) with no targetId (0),
so channel messages went nowhere; now passes session.currentChannelId.

Fix iOS per-user tuning for VoiceOver: the tuning sheet was long-press
.contextMenu only (invisible to VoiceOver); UserRow now also exposes the same
buttons via .accessibilityActions (no visual change).

Verified: core builds clean; ctest 24/27 (3 pre-existing teardown crashes,
reproduced with changes stashed); VoiceCatMac + VoiceCatiOS (arm64 sim) build
SUCCEEDED; VoiceCat.Interop dotnet build succeeded. Windows App not built
(WinForms can't build on macOS) — follows existing patterns.

clients/apple/macOS/VoiceCatMac/Windows/MainWindowController.swift

@@ -33,7 +33,9 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
     private var screenCapture: ScreenAudioCapture?
     // Last app/exclusion choice from the share picker; reused as the default next time.
     private var screenAudioSelection: ScreenAudioSelection = .default
-    internal var pttKeyCode: UInt16 = 0x60  // F8
+    internal var pttKeyCode: UInt16 = 0x60 {  // F8
+        didSet { UserDefaults.standard.set(Int(pttKeyCode), forKey: AudioDefaults.pttKeyCode) }
+    }
     private var pttMonitor: Any?
     private var pttEngaged = false  // guards the PTT cue against key-repeat
     private var serverMuted = false
@@ -49,9 +51,26 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
     private var settingsWindowController: SettingsWindowController?
 
     // MARK: - Audio settings state (source of truth — read/written by SettingsWindowController)
+    // The input mode / VAD threshold / mic gain / PTT key persist via UserDefaults (didSet below)
+    // so they survive relaunch; loadPersistedAudioSettings() restores them at startup and they are
+    // pushed into the core when the mic stream starts (micToggleClicked).
 
-    internal var selectedInputMode: VoiceCatInputMode = .voiceActivation
-    internal var vadThresholdValue: Float = 0.05
+    enum AudioDefaults {
+        static let inputMode    = "voice.inputMode"
+        static let vadThreshold = "voice.vadThreshold"
+        static let inputGain    = "voice.inputGain"
+        static let pttKeyCode   = "voice.pttKeyCode"
+    }
+
+    internal var selectedInputMode: VoiceCatInputMode = .voiceActivation {
+        didSet { UserDefaults.standard.set(Int(selectedInputMode.rawValue), forKey: AudioDefaults.inputMode) }
+    }
+    internal var vadThresholdValue: Float = 0.05 {
+        didSet { UserDefaults.standard.set(vadThresholdValue, forKey: AudioDefaults.vadThreshold) }
+    }
+    internal var inputGain: Float = 1.0 {
+        didSet { UserDefaults.standard.set(inputGain, forKey: AudioDefaults.inputGain) }
+    }
     internal var selectedInputDeviceId: String?
 
     // MARK: - UI components
@@ -97,12 +116,32 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
         super.init(window: window)
         window.delegate = self
 
+        loadPersistedAudioSettings()
         buildUI()
         buildToolbar()
         wireEvents()
         bootstrap()
     }
 
+    /// Restore the saved input mode / VAD threshold / mic gain / PTT key from UserDefaults so a
+    /// relaunch keeps the user's transmission settings instead of resetting to VAD defaults.
+    private func loadPersistedAudioSettings() {
+        let d = UserDefaults.standard
+        if d.object(forKey: AudioDefaults.inputMode) != nil {
+            let raw = UInt32(d.integer(forKey: AudioDefaults.inputMode))
+            selectedInputMode = VoiceCatInputMode(rawValue: raw) ?? .voiceActivation
+        }
+        if d.object(forKey: AudioDefaults.vadThreshold) != nil {
+            vadThresholdValue = d.float(forKey: AudioDefaults.vadThreshold)
+        }
+        if d.object(forKey: AudioDefaults.inputGain) != nil {
+            inputGain = d.float(forKey: AudioDefaults.inputGain)
+        }
+        if d.object(forKey: AudioDefaults.pttKeyCode) != nil {
+            pttKeyCode = UInt16(d.integer(forKey: AudioDefaults.pttKeyCode))
+        }
+    }
+
     required init?(coder: NSCoder) { fatalError() }
 
     deinit {
@@ -712,6 +751,7 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
                 if selectedInputMode == .voiceActivation {
                     client.setVadThreshold(vadThresholdValue)
                 }
+                client.setInputGain(inputGain)
                 setVoiceJoinedState(true)
                 addActivity("Joined voice — microphone active")
                 EventFeedback.shared.play(.voiceOn)