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/macOS/VoiceCatMac/Sheets/AddServerSheet.swift

@@ -13,6 +13,7 @@ final class AddServerSheet: NSViewController {
         return f
     }()
     private let authPicker = NSPopUpButton()
+    private let nicknameField = NSTextField()
     private let usernameField = NSTextField()
     private let passwordField = NSSecureTextField()
     private let savePwCheckbox = NSButton(checkboxWithTitle: "Save password in Keychain", target: nil, action: nil)
@@ -35,6 +36,7 @@ final class AddServerSheet: NSViewController {
             hostField.stringValue = s.host
             portField.stringValue = "\(s.port)"
             authPicker.selectItem(withTitle: s.authMode == .guest ? "Guest" : "Account")
+            nicknameField.stringValue = s.nickname ?? ""
             usernameField.stringValue = s.savedUsername ?? ""
             updateAuthVisibility()
         }
@@ -47,6 +49,7 @@ final class AddServerSheet: NSViewController {
         let hostLabel = NSTextField(labelWithString: "Host:")
         let portLabel = NSTextField(labelWithString: "Port:")
         let authLabel = NSTextField(labelWithString: "Auth:")
+        let nickLabel = NSTextField(labelWithString: "Nickname:")
         let userLabel = NSTextField(labelWithString: "Username:")
         let pwLabel = NSTextField(labelWithString: "Password:")
 
@@ -60,6 +63,9 @@ final class AddServerSheet: NSViewController {
         authPicker.target = self; authPicker.action = #selector(authChanged)
         authPicker.setAccessibilityLabel("Authentication mode")
 
+        nicknameField.placeholderString = "leave blank to use your system name"
+        nicknameField.setAccessibilityLabel("Guest nickname (display name)")
+
         usernameField.placeholderString = "username"
         usernameField.setAccessibilityLabel("Username")
 
@@ -79,6 +85,7 @@ final class AddServerSheet: NSViewController {
             [hostLabel, hostField],
             [portLabel, portField],
             [authLabel, authPicker],
+            [nickLabel, nicknameField],
             [userLabel, usernameField],
             [pwLabel, passwordField],
             [NSView(), savePwCheckbox],
@@ -110,6 +117,7 @@ final class AddServerSheet: NSViewController {
 
     private func updateAuthVisibility() {
         let isAccount = authPicker.titleOfSelectedItem == "Account"
+        nicknameField.isEnabled = !isAccount
         usernameField.isEnabled = isAccount
         passwordField.isEnabled = isAccount
         savePwCheckbox.isEnabled = isAccount
@@ -130,6 +138,8 @@ final class AddServerSheet: NSViewController {
         server.host = host; server.port = port
         server.authMode = isGuest ? .guest : .password
         server.savedUsername = isGuest ? nil : usernameField.stringValue.trimmingCharacters(in: .whitespaces)
+        let nick = nicknameField.stringValue.trimmingCharacters(in: .whitespaces)
+        server.nickname = nick.isEmpty ? nil : nick
 
         if !isGuest && savePwCheckbox.state == .on {
             let pw = passwordField.stringValue