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/Models.swift

@@ -193,15 +193,16 @@ public struct AudioConfig: Sendable, Equatable {
     public let expectedPacketLoss: UInt32   // % 0..100
     public let dtx: Bool
     public let complexity: UInt32       // 0..10
+    public let dred: Bool               // Deep REDundancy (Opus 1.6), off by default
 
     public init(codec: UInt32 = 0, stereo: Bool = false, sampleRate: UInt32 = 48000,
                 bitrateBps: UInt32 = 64000, frameMs: UInt32 = 20, application: UInt32 = 0,
                 fec: Bool = true, expectedPacketLoss: UInt32 = 5, dtx: Bool = false,
-                complexity: UInt32 = 10) {
+                complexity: UInt32 = 10, dred: Bool = false) {
         self.codec = codec; self.stereo = stereo; self.sampleRate = sampleRate
         self.bitrateBps = bitrateBps; self.frameMs = frameMs; self.application = application
         self.fec = fec; self.expectedPacketLoss = expectedPacketLoss; self.dtx = dtx
-        self.complexity = complexity
+        self.complexity = complexity; self.dred = dred
     }
 }