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/Sources/VoiceCatCore/Marshaling.swift

@@ -94,7 +94,7 @@ internal enum Marshaling {
         AudioConfig(codec: c.codec, stereo: c.mode != 0, sampleRate: c.sample_rate,
                     bitrateBps: c.bitrate_bps, frameMs: c.frame_ms, application: c.application,
                     fec: c.fec != 0, expectedPacketLoss: c.expected_packet_loss,
-                    dtx: c.dtx != 0, complexity: c.complexity)
+                    dtx: c.dtx != 0, complexity: c.complexity, dred: c.dred != 0)
     }
 
     static func permissions(_ p: vc_permissions) -> Permissions {