feat(macos): UI overhaul -- toolbar, unified log, PM windows, settings window, hotkeys

Mirrors the Windows client's UI overhaul (commit 97fa659 + 540ec13) adapted to
Mac-native conventions. The main window is now just toolbar + channels + users
+ chat; audio device settings moved to a modeless Settings window.

- NSToolbar: Join Voice, Share Screen Audio, Mute, Deafen (SF Symbol toggle
  buttons) + Output Volume slider (NSSlider 0-100, default 80). Voice actions,
  mute/deafen, and output volume moved out of the bottom panel into the toolbar
- Audio device settings (input mode, VAD sensitivity, PTT key, device picker,
  level meter) moved to a new SettingsWindowController -- a modeless window
  opened via the app menu's "Settings..." (Cmd+,) item. Source-of-truth for
  audio state lives in MainWindowController so voice start applies settings even
  before the window has been opened; SettingsWindowController reads from /
  writes back to those properties and applies changes live when voice is active.
  Level meter forwarded from handleLevel -> updateLevel(rms:)
- Unified log: chat NSTextView + activity NSTableView collapsed into a single
  NSTextView -- activity events in secondaryLabelColor (gray), chat in default
- Private messaging: scope dropdown removed; compose always sends to the
  current channel. Each PM conversation opens in its own modeless
  PrivateMessageWindowController. Incoming .textMessage with .private scope
  routed to the right window; outgoing PMs echoed by server arrive through the
  same path. "Send Private Message..." added to user context menu.
- Messages menu: "New Private Message..." (Cmd+Shift+N) opens a UserPickerSheet
  listing all server users so you can PM anyone on the server
- Channel tree now shows live user counts, e.g. "General (3)"; refreshChannelTree
  called on .userJoined/.userLeft (was missing)
- Voice menu: Join Voice (Cmd+Shift+V), Share Screen Audio (Cmd+Shift+S),
  Mute (Cmd+Shift+M), Deafen (Cmd+Shift+D) -- NSMenuItem key equivalents with
  [.command, .shift] mask, dispatched by the responder chain
- setOutputVolume(_:) wrapper added to VoiceCatClient.swift (was missing -- the
  C ABI + C# wrapper shipped in commit 97fa659 but the Swift wrapper was never
  added); wired end-to-end: toolbar slider -> client.setOutputVolume(gain)

Part A -- fixed and verified the previously-uncompiled Swift from the external
PCM feed/tap commit (615d2a8):
- Rebuilt the macOS xcframework slice (regenerated the module map from current
  voicecat.h, exposing vc_pcm_sink_cb / vc_stream_feed_pcm / vc_set_pcm_sink)
- Fixed feedPcm type bug: size_t imports as Int in Swift not UInt; the original
  UInt(samplesPerChannel) was wrong
- Added VoiceCatPcmSinkCallback typealias -- a Swift-idiomatic public alias for
  the C vc_pcm_sink_cb so consumers (tests, the macOS app) can declare a sink
  callback without directly importing the VoiceCatC C module. Mirrors the C#
  VcPcmSinkCallback delegate
- keyCodeName helper deduplicated (was in PttKeyCaptureSheet.swift +
  MainWindowController.swift -- now shared)

Platform-specific adaptations (vs. Windows): NSToolbar instead of ToolStrip;
global menu bar + NSMenuItem key equivalents (Cmd not Ctrl, responder-chain
dispatched, no custom key monitor needed); PM windows as modeless NSWindows;
picker as Mac sheet; gray = secondaryLabelColor; SF Symbols for toolbar icons.

swift test 10/10 (4 ExternalPcmTests + 6 VoiceCatClientSmokeTests against a
live server); xcodebuild Debug + Release BUILD SUCCEEDED with 0 Swift warnings.

clients/apple/Sources/VoiceCatCore/VoiceCatClient.swift

@@ -35,6 +35,12 @@
 import VoiceCatC
 import Foundation
 
+/// Swift-idiomatic alias for the C `vc_pcm_sink_cb` function-pointer type from
+/// `voicecat.h`. Exposed publicly so consumers (`VoiceCatMac`, tests) can declare a sink
+/// callback without directly importing the `VoiceCatC` C module. Mirrors the C# wrapper's
+/// `VcPcmSinkCallback` delegate.
+public typealias VoiceCatPcmSinkCallback = vc_pcm_sink_cb
+
 /// The Swift wrapper around `vc_client*`. Owns the native handle for its entire lifetime;
 /// `deinit` destroys it. Events and level meters are delivered on the main queue via the
 /// `onEvent` / `onLevel` closures.
@@ -324,7 +330,7 @@ public final class VoiceCatClient {
     public func feedPcm(streamId: UInt32, pcm: UnsafePointer<Int16>,
                         samplesPerChannel: Int, channels: UInt32) -> VoiceCatResult {
         VoiceCatResult(vc_stream_feed_pcm(handle, streamId, pcm,
-                                         UInt(samplesPerChannel), channels))
+                                          samplesPerChannel, channels))
     }
 
     /// Convenience overload for feeding from a Swift `[Int16]` array.
@@ -345,7 +351,7 @@ public final class VoiceCatClient {
     ///
     /// Pass `nil` to disable (default). The callback MUST NOT block or allocate.
     @discardableResult
-    public func setPcmSink(_ cb: vc_pcm_sink_cb?, user: UnsafeMutableRawPointer?) -> VoiceCatResult {
+    public func setPcmSink(_ cb: VoiceCatPcmSinkCallback?, user: UnsafeMutableRawPointer?) -> VoiceCatResult {
         VoiceCatResult(vc_set_pcm_sink(handle, cb, user))
     }
 
@@ -370,6 +376,14 @@ public final class VoiceCatClient {
         VoiceCatResult(vc_set_self_mute(handle, micMuted ? 1 : 0, deafened ? 1 : 0))
     }
 
+    /// Global playback volume applied after mixing all remote streams. gain 0.0 = silent,
+    /// 1.0 = unity (default), >1.0 amplifies. Always LOCAL — no protocol traffic. Mirrors the
+    /// Windows client's `SetOutputVolume` and the C ABI `vc_set_output_volume` added in M5.
+    @discardableResult
+    public func setOutputVolume(_ gain: Float) -> VoiceCatResult {
+        VoiceCatResult(vc_set_output_volume(handle, gain < 0 ? 0 : gain))
+    }
+
     // MARK: - AVAudioSession interruption hooks (iOS)
 
     /// Pause miniaudio device I/O. Call when AVAudioSession interruption begins.