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/apple/iOS/VoiceCatiOS/Views/ChannelBrowserView.swift

@@ -7,6 +7,7 @@ import VoiceCatCore
 struct ChannelBrowserView: View {
     @Bindable var session: SessionState
     @State private var showCreateChannel = false
+    @State private var editChannel: Channel?
 
     var body: some View {
         NavigationStack {
@@ -26,6 +27,16 @@ struct ChannelBrowserView: View {
                             }
                         }
                     }
+                    .swipeActions(edge: .leading) {
+                        if session.permissions.isAdmin {
+                            Button {
+                                editChannel = ch
+                            } label: {
+                                Label("Edit", systemImage: "pencil")
+                            }
+                            .tint(.blue)
+                        }
+                    }
                 }
             }
             .navigationTitle("Channels")
@@ -44,6 +55,9 @@ struct ChannelBrowserView: View {
             .sheet(isPresented: $showCreateChannel) {
                 ChannelEditView(channelId: nil, session: session)
             }
+            .sheet(item: $editChannel) { ch in
+                ChannelEditView(channelId: ch.id, session: session)
+            }
         }
     }