feat(macos): VoiceCatMac AppKit client + fix xcodebuild

The previous "shipped" claim was false — xcodebuild had never been run and
the macOS app source was never committed. This commit adds the 17 Swift
source files + xcodeproj and fixes three real defect classes so Debug and
Release both build clean:

1. MainWindowController.swift compile errors:
   - NSAccessibility.post arg order (element:notification:userInfo:)
   - NSAccessibilityPriorityMedium -> NSAccessibilityPriorityLevel.medium
   - StreamSummary.streamId -> .id (Identifiable conformance)
   - drop redundant VoiceCatResult.description extension
2. Linker: add -lc++ to OTHER_LDFLAGS (libvoicecat-fat.a is C++20; pure-Swift
   app target has no .cpp sources so libc++ wasn't pulled in — swift test
   passed because Package.swift testTarget has linkerSettings: c++).
3. Release config: add ONLY_ACTIVE_ARCH=YES (XCFramework only has arm64).

Verified: clean Debug + Release builds, otool -L shows libc++.1.dylib,
nm shows _vc_client_create/_vc_version_string, app launches and runs.

clients/apple/macOS/VoiceCatMac/Sheets/MoveUserSheet.swift

@@ -0,0 +1,73 @@
+import AppKit
+import VoiceCatCore
+
+final class MoveUserSheet: NSViewController {
+
+    var onComplete: ((UInt32?) -> Void)?
+
+    private let channels: [Channel]
+    private let currentChannelId: UInt32
+    private let picker = NSPopUpButton()
+
+    init(channels: [Channel], currentChannelId: UInt32) {
+        self.channels = channels.sorted(by: { $0.name < $1.name })
+        self.currentChannelId = currentChannelId
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 300, height: 100))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        buildUI()
+    }
+
+    private func buildUI() {
+        let label = NSTextField(labelWithString: "Move user to channel:")
+        label.setAccessibilityLabel("Select destination channel")
+
+        for ch in channels where ch.id != currentChannelId {
+            let item = NSMenuItem(title: ch.name, action: nil, keyEquivalent: "")
+            item.representedObject = ch.id
+            picker.menu?.addItem(item)
+        }
+        picker.setAccessibilityLabel("Destination channel")
+
+        let cancelButton = NSButton(title: "Cancel", target: self, action: #selector(cancelClicked))
+        cancelButton.bezelStyle = .rounded
+
+        let moveButton = NSButton(title: "Move", target: self, action: #selector(moveClicked))
+        moveButton.bezelStyle = .rounded; moveButton.keyEquivalent = "\r"
+        moveButton.setAccessibilityLabel("Move user to selected channel")
+
+        let buttonRow = NSStackView(views: [NSView(), cancelButton, moveButton])
+        buttonRow.orientation = .horizontal; buttonRow.spacing = 8
+
+        let stack = NSStackView(views: [label, picker, buttonRow])
+        stack.orientation = .vertical; stack.spacing = 10
+        stack.edgeInsets = NSEdgeInsets(top: 16, left: 16, bottom: 16, right: 16)
+        stack.translatesAutoresizingMaskIntoConstraints = false
+        view.addSubview(stack)
+        NSLayoutConstraint.activate([
+            stack.topAnchor.constraint(equalTo: view.topAnchor),
+            stack.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            stack.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+            stack.bottomAnchor.constraint(equalTo: view.bottomAnchor),
+        ])
+    }
+
+    @objc private func moveClicked() {
+        let channelId = picker.selectedItem?.representedObject as? UInt32
+        dismiss(nil)
+        onComplete?(channelId)
+    }
+
+    @objc private func cancelClicked() {
+        dismiss(nil)
+        onComplete?(nil)
+    }
+}