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/windows/VoiceCat.App/Models/VoiceSettings.cs

@@ -0,0 +1,53 @@
+using System.Text.Json;
+
+namespace VoiceCat.App.Models;
+
+/// <summary>
+/// User's send-side voice input preferences — transmission mode, VAD sensitivity, mic input
+/// gain, and the push-to-talk key. Persisted to %AppData%\VoiceCat\voice.json, same pattern as
+/// <see cref="ServerListStore"/> and FeedbackSettings: a missing or corrupt file yields defaults
+/// rather than throwing. Slider-position values are stored as-is so MainForm can restore the
+/// TrackBars directly.
+/// </summary>
+public sealed class VoiceSettings
+{
+    /// <summary>Transmission mode: 0 = voice activation, 1 = push-to-talk, 2 = always on
+    /// (matches Interop's VcInputMode).</summary>
+    public int InputMode { get; set; } = 0;
+
+    /// <summary>VAD sensitivity slider position, 1–100 (default mirrors the designer's 76).</summary>
+    public int VadThresholdSlider { get; set; } = 76;
+
+    /// <summary>Microphone input gain slider position, 0–300 percent (100 = unity).</summary>
+    public int MicGain { get; set; } = 100;
+
+    /// <summary>Push-to-talk key, stored as the integer value of System.Windows.Forms.Keys.</summary>
+    public int PttKey { get; set; } = (int)Keys.F8;
+
+    private static readonly JsonSerializerOptions JsonOptions = new() { WriteIndented = true };
+
+    private static string AppDataDir => Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "VoiceCat");
+
+    private static string FilePath => Path.Combine(AppDataDir, "voice.json");
+
+    public static VoiceSettings Load()
+    {
+        try
+        {
+            if (!File.Exists(FilePath)) return new VoiceSettings();
+            string json = File.ReadAllText(FilePath);
+            return JsonSerializer.Deserialize<VoiceSettings>(json) ?? new VoiceSettings();
+        }
+        catch
+        {
+            return new VoiceSettings();
+        }
+    }
+
+    public void Save()
+    {
+        Directory.CreateDirectory(AppDataDir);
+        File.WriteAllText(FilePath, JsonSerializer.Serialize(this, JsonOptions));
+    }
+}