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/SettingsWindowController.swift

@@ -50,6 +50,14 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
     // Cached VAD slider position so we can restore it when the window reopens.
     private var vadSliderValue: Double = 50
 
+    // Mic input gain: 0–300 % (100 = unity). Persisted via MainWindowController.inputGain.
+    private let inputGainSlider: NSSlider = {
+        let s = NSSlider(value: 100, minValue: 0, maxValue: 300, target: nil, action: nil)
+        s.numberOfTickMarks = 0
+        return s
+    }()
+    private let inputGainValueLabel = NSTextField(labelWithString: "100%")
+
     // Notification feedback controls. Read/write UserDefaults with the same keys VoiceCatCore's
     // FeedbackSettings reads, so EventFeedback honours these immediately.
     private let soundsCheckbox = NSButton(checkboxWithTitle: "Event sounds", target: nil, action: nil)
@@ -135,6 +143,14 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         levelMeter.setAccessibilityLabel("Microphone input level")
         levelMeter.setAccessibilityHelp("Shows current microphone volume level")
 
+        let inputGainLabel = NSTextField(labelWithString: "Mic volume:")
+        inputGainLabel.setAccessibilityLabel("Microphone volume")
+        inputGainSlider.target = self
+        inputGainSlider.action = #selector(inputGainChanged)
+        inputGainSlider.setAccessibilityLabel("Microphone volume")
+        inputGainSlider.setAccessibilityHelp("Boost a quiet microphone. 100% is unity.")
+        inputGainValueLabel.setAccessibilityLabel("Microphone volume value")
+
         let inputModeRow = NSStackView(views: [inputModeLabel, inputModeControl])
         inputModeRow.orientation = .horizontal
         inputModeRow.spacing = 8
@@ -155,6 +171,10 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         levelRow.orientation = .horizontal
         levelRow.spacing = 8
 
+        let inputGainRow = NSStackView(views: [inputGainLabel, inputGainSlider, inputGainValueLabel])
+        inputGainRow.orientation = .horizontal
+        inputGainRow.spacing = 8
+
         // Notifications
         let notificationsHeader = NSTextField(labelWithString: "Notifications")
         notificationsHeader.font = .boldSystemFont(ofSize: NSFont.systemFontSize)
@@ -177,8 +197,8 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         volumeRow.orientation = .horizontal
         volumeRow.spacing = 8
 
-        let stack = NSStackView(views: [inputModeRow, vadRow, pttRow, deviceRow, levelRow,
-                                        notificationsHeader, soundsCheckbox, volumeRow,
+        let stack = NSStackView(views: [inputModeRow, vadRow, inputGainRow, pttRow, deviceRow,
+                                        levelRow, notificationsHeader, soundsCheckbox, volumeRow,
                                         speechCheckbox, selfTalkCheckbox, pttSoundCheckbox])
         stack.orientation = .vertical
         stack.spacing = 12
@@ -194,6 +214,7 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
             stack.bottomAnchor.constraint(equalTo: contentView.bottomAnchor),
 
             vadSlider.widthAnchor.constraint(greaterThanOrEqualToConstant: 200),
+            inputGainSlider.widthAnchor.constraint(greaterThanOrEqualToConstant: 180),
             levelMeter.widthAnchor.constraint(equalToConstant: 200),
             devicePicker.widthAnchor.constraint(greaterThanOrEqualToConstant: 180),
             soundsVolumeSlider.widthAnchor.constraint(greaterThanOrEqualToConstant: 200),
@@ -235,9 +256,15 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         case .alwaysOn:        inputModeControl.selectedSegment = 2
         }
 
+        // Restore the slider from the persisted threshold (invert vadThresholdFromSlider) so a
+        // relaunch shows the saved sensitivity, not the default mid-point.
+        vadSliderValue = vadSliderFromThreshold(mc.vadThresholdValue)
         vadSlider.doubleValue = vadSliderValue
         pttKeyLabel.stringValue = "(\(keyCodeName(mc.pttKeyCode)))"
 
+        inputGainSlider.doubleValue = Double(mc.inputGain * 100)
+        updateInputGainLabel()
+
         updateConditionalControls()
     }
 
@@ -275,6 +302,21 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         }
     }
 
+    @objc private func inputGainChanged() {
+        let gain = Float(inputGainSlider.doubleValue) / 100.0
+        mainController?.inputGain = gain
+        updateInputGainLabel()
+        if let mc = mainController, mc.micStreamId != 0 {
+            client.setInputGain(gain)
+        }
+    }
+
+    private func updateInputGainLabel() {
+        let pct = Int(inputGainSlider.doubleValue.rounded())
+        inputGainValueLabel.stringValue = "\(pct)%"
+        inputGainSlider.setAccessibilityValue("\(pct) percent")
+    }
+
     @objc private func changePttClicked() {
         guard let mc = mainController else { return }
         let sheet = PttKeyCaptureSheet(currentKeyCode: mc.pttKeyCode)
@@ -346,6 +388,12 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         0.1 * (1.0 - Float(vadSlider.doubleValue - 1.0) / 99.0)
     }
 
+    /// Inverse of vadThresholdFromSlider: map a stored threshold back to a 1…100 slider position.
+    private func vadSliderFromThreshold(_ threshold: Float) -> Double {
+        let clamped = min(max(threshold, 0.0), 0.1)
+        return Double(1.0 + (1.0 - clamped / 0.1) * 99.0)
+    }
+
     private func presentSheet(_ vc: NSViewController) {
         if let cvc = window?.contentViewController {
             cvc.presentAsSheet(vc)