feat(apple): screen-audio sharing -- macOS ScreenCaptureKit, iOS ReplayKit

Implement system/desktop audio sharing on the Apple clients, feeding the
existing SCREEN_AUDIO Opus -> AEAD -> UDP path via vc_stream_feed_pcm. No
C++/protocol/codec changes -- the core was already ready (the Windows-only
loopback is #ifdef VOICECAT_HAS_LOOPBACK; off Windows the stream just waits
for fed PCM). Audio only; video is dropped.

macOS (in-process):
- ScreenAudioCapture.swift drives an audio-only SCStream
  (excludesCurrentProcessAudio), converts Float32 -> int16 in the channel's
  mono/stereo mode, and calls feedPcm. Capture starts on the self
  .streamStarted event (effective config known then). Wired into
  MainWindowController.screenAudioClicked().

iOS (forward-to-host, single session):
- VoiceCatBroadcast: a ReplayKit Broadcast Upload Extension consumes
  .audioApp only, resamples to 48kHz int16 stereo (AVAudioConverter), and
  writes a shared App Group SPSC ring (BroadcastAudioRing.swift). It does
  not link libvoicecat.
- Host BroadcastAudioPump drains the ring (reacting to the extension's
  Darwin notifications) and feeds the SCREEN_AUDIO stream it owns, downmixing
  to mono when the channel is mono. Screen audio appears as a second stream
  of the same user; no credentials persisted. UI is RPSystemBroadcastPicker
  View in VoiceControlsView. Removes the speculative BroadcastCredentials.

Docs: voice.md s9, CLAUDE.md status, PROGRESS.md.

clients/apple/iOS/VoiceCatiOS/ServerListStore.swift

@@ -89,18 +89,4 @@ final class ServerListStore {
         SecItemDelete(query as CFDictionary)
     }
 
-    // MARK: - Broadcast credentials (shared with ReplayKit extension)
-
-    func writeBroadcastCredentials(server: SavedServer, nickname: String?) {
-        let creds = BroadcastCredentials(
-            host: server.host,
-            port: Int(server.port),
-            authMode: server.authMode.rawValue,
-            username: server.authMode == .password ? server.savedUsername : (nickname ?? ""),
-            tofuPinsPath: tofuStorePath
-        )
-        guard let data = try? JSONEncoder().encode(creds) else { return }
-        let url = voicecatDir.appendingPathComponent("broadcast_credentials.json")
-        try? data.write(to: url, options: .atomic)
-    }
 }