feat(clients): expose all channel codec params + guest nickname everywhere

Channel create/edit UIs only surfaced a subset of the core's vc_audio_config,
and DRED was exposed nowhere. While adding it, found a latent ABI mismatch:
both Swift AudioConfig and the C# VcAudioConfigNative blittable struct were one
int short of the native vc_audio_config (missing the trailing `dred`), so native
read past the managed struct in vc_create_channel/vc_edit_channel.

- core marshaling: thread `dred` through Swift (Models/Marshaling/toNative) and
  C# (Structs/Models/Marshaling/VoiceCatClient) -- fixes the ABI gap + enables it
- windows: add the one missing DRED checkbox to ChannelEditDialog
- macos: ChannelEditSheet now exposes application, sample rate, packet loss,
  complexity, and DRED (was stereo/bitrate/frame/FEC/DTX only)
- ios: rebuild ChannelEditView into a full create+edit form (all params); add
  SessionState.editChannel + an admin Edit swipe action (iOS had no edit UI)
- guest nickname: add a dedicated `nickname` to SavedServer on macOS+iOS
  (backward-compatible Codable), shown in Guest mode, wired into the guest auth
  path -- guests could not set a display name on either before (only Windows)

Verified: macOS + iOS (sim, arm64) xcodebuild BUILD SUCCEEDED; core ctest 22/23
(only external_pcm aborts on a pre-existing shutdown mutex race; no C++ changed).

clients/windows/VoiceCat.App/Forms/ChannelEditDialog.cs

@@ -28,6 +28,7 @@ public sealed class ChannelEditDialog : Form
     private CheckBox _chkFec = null!;
     private NumericUpDown _numExpectedLoss = null!;
     private CheckBox _chkDtx = null!;
+    private CheckBox _chkDred = null!;
     private NumericUpDown _numComplexity = null!;
 
     public ChannelEditInfo? Result { get; private set; }
@@ -183,7 +184,7 @@ public sealed class ChannelEditDialog : Form
 
     private void BuildAudioPage(TabPage page, AudioConfigInfo? audio)
     {
-        audio ??= new AudioConfigInfo(0, false, 48000, 0, 20, 0, true, 0, false, 10);
+        audio ??= new AudioConfigInfo(0, false, 48000, 0, 20, 0, true, 0, false, 10, false);
 
         int y = 16;
         int labelWidth = 150;
@@ -314,6 +315,17 @@ public sealed class ChannelEditDialog : Form
             TabIndex = 18,
         };
         page.Controls.Add(_chkDtx);
+        y += 28;
+
+        _chkDred = new CheckBox
+        {
+            Text = "D&RED (deep redundancy)",
+            Location = new Point(inputX, y),
+            AutoSize = true,
+            Checked = audio.Dred,
+            TabIndex = 19,
+        };
+        page.Controls.Add(_chkDred);
     }
 
     private static void AddLabel(Control parent, string text, int x, int y, int width)
@@ -362,7 +374,8 @@ public sealed class ChannelEditDialog : Form
             Fec: _chkFec.Checked,
             ExpectedPacketLoss: (uint)_numExpectedLoss.Value,
             Dtx: _chkDtx.Checked,
-            Complexity: (uint)_numComplexity.Value);
+            Complexity: (uint)_numComplexity.Value,
+            Dred: _chkDred.Checked);
 
         Result = new ChannelEditInfo(
             Id: _editingId,