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/SessionState.swift

@@ -342,13 +342,14 @@ final class SessionState {
         client.setServerMute(userId, muted: muted, deafened: deafened)
     }
 
-    func createChannel(name: String, topic: String) {
-        let info = ChannelEdit(id: 0, parentId: 0, name: name, topic: topic,
-                               passwordProtected: false, password: nil,
-                               maxUsers: 0, sortOrder: 0, audio: AudioConfig())
+    func createChannel(_ info: ChannelEdit) {
         client.createChannel(info)
     }
 
+    func editChannel(_ info: ChannelEdit) {
+        client.editChannel(info)
+    }
+
     func deleteChannel(_ channelId: UInt32) {
         client.deleteChannel(channelId)
     }