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/AddServerView.swift

@@ -9,6 +9,7 @@ struct AddServerView: View {
     @State private var host = ""
     @State private var port = "7878"
     @State private var authMode = SavedServer.AuthMode.guest
+    @State private var nickname = ""
     @State private var username = ""
     @State private var password = ""
     @State private var savePassword = false
@@ -35,6 +36,13 @@ struct AddServerView: View {
                     .pickerStyle(.segmented)
                     .accessibilityLabel("Authentication mode")
 
+                    if authMode == .guest {
+                        TextField("Nickname (optional)", text: $nickname)
+                            .autocorrectionDisabled()
+                            .textInputAutocapitalization(.never)
+                            .accessibilityLabel("Guest nickname, optional display name")
+                    }
+
                     if authMode == .password {
                         TextField("Username", text: $username)
                             .textContentType(.username)
@@ -67,6 +75,7 @@ struct AddServerView: View {
                 port = "\(s.port)"
                 authMode = s.authMode
                 username = s.savedUsername
+                nickname = s.nickname ?? ""
             }
         }
     }
@@ -76,17 +85,21 @@ struct AddServerView: View {
         guard !trimmedHost.isEmpty, let portNum = UInt16(port) else { return }
 
         let pw = (savePassword && authMode == .password && !password.isEmpty) ? password : nil
+        let trimmedNick = nickname.trimmingCharacters(in: .whitespaces)
+        let nick: String? = (authMode == .guest && !trimmedNick.isEmpty) ? trimmedNick : nil
 
         if var s = editing {
             s.host = trimmedHost
             s.port = portNum
             s.authMode = authMode
             s.savedUsername = authMode == .password ? username : ""
+            s.nickname = nick
             appState.updateServer(s, password: pw)
         } else {
             let s = SavedServer(host: trimmedHost, port: portNum,
                                 authMode: authMode,
-                                savedUsername: authMode == .password ? username : "")
+                                savedUsername: authMode == .password ? username : "",
+                                nickname: nick)
             appState.addServer(s, password: pw)
         }
         dismiss()