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/macOS/VoiceCatMac/Sheets/PttKeyCaptureSheet.swift

@@ -101,13 +101,3 @@ private final class KeyCaptureView: NSView {
     }
     override var focusRingMaskBounds: NSRect { bounds }
 }
-
-private func keyCodeName(_ keyCode: UInt16) -> String {
-    let map: [UInt16: String] = [
-        0x60: "F5", 0x61: "F6", 0x62: "F7", 0x63: "F3", 0x64: "F8", 0x65: "F9",
-        0x67: "F11", 0x69: "F13", 0x6A: "F16", 0x6B: "F14", 0x6D: "F10", 0x6F: "F12",
-        0x71: "F15", 0x72: "Help", 0x73: "Home", 0x74: "PgUp", 0x75: "Del",
-        0x76: "F4", 0x77: "End", 0x78: "F2", 0x79: "PgDn", 0x7A: "F1",
-    ]
-    return map[keyCode] ?? "Key\(keyCode)"
-}