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/Forms/MainForm.Designer.cs

@@ -51,6 +51,8 @@ partial class MainForm
     private ProgressBar pbLevel = null!;
     private Label lblVadThreshold = null!;
     private TrackBar trkVadThreshold = null!;
+    private Label lblMicGain = null!;
+    private TrackBar trkMicGain = null!;
 
     protected override void Dispose(bool disposing)
     {
@@ -94,6 +96,8 @@ partial class MainForm
         pbLevel = new ProgressBar();
         lblVadThreshold = new Label();
         trkVadThreshold = new TrackBar();
+        lblMicGain = new Label();
+        trkMicGain = new TrackBar();
         menuStrip = new MenuStrip();
         toolStrip = new ToolStrip();
         tsbJoinVoice = new ToolStripButton();
@@ -336,6 +340,23 @@ partial class MainForm
         trkVadThreshold.TabIndex = 9;
         trkVadThreshold.Visible = true;
 
+        lblMicGain.Text = "Mic volume:";
+        lblMicGain.AutoSize = true;
+        lblMicGain.Margin = new Padding(12, 5, 4, 0);
+
+        trkMicGain.AccessibleName = "Microphone volume";
+        trkMicGain.AccessibleDescription =
+            "Boost a quiet microphone. 100 is unity; range 0–300 percent.";
+        trkMicGain.Minimum = 0;
+        trkMicGain.Maximum = 300;
+        trkMicGain.Value = 100;
+        trkMicGain.TickFrequency = 25;
+        trkMicGain.SmallChange = 5;
+        trkMicGain.LargeChange = 25;
+        trkMicGain.Width = 120;
+        trkMicGain.Margin = new Padding(0, 2, 0, 0);
+        trkMicGain.TabIndex = 10;
+
         flpVoiceBottom.Dock = DockStyle.Fill;
         flpVoiceBottom.Padding = new Padding(4, 0, 4, 2);
         flpVoiceBottom.Controls.Add(lblInputDevice);
@@ -345,6 +366,8 @@ partial class MainForm
         flpVoiceBottom.Controls.Add(pbLevel);
         flpVoiceBottom.Controls.Add(lblVadThreshold);
         flpVoiceBottom.Controls.Add(trkVadThreshold);
+        flpVoiceBottom.Controls.Add(lblMicGain);
+        flpVoiceBottom.Controls.Add(trkMicGain);
 
         pnlVoice.Dock = DockStyle.Bottom;
         pnlVoice.Height = 68;