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/PermissionsSheet.swift

@@ -0,0 +1,100 @@
+import AppKit
+import VoiceCatCore
+
+final class PermissionsSheet: NSViewController {
+
+    var onComplete: ((Permissions?) -> Void)?
+
+    private let targetNickname: String
+    private let initial: Permissions
+
+    private let canCreateTempChannelCB = NSButton(checkboxWithTitle: "Create temporary channels", target: nil, action: nil)
+    private let canKickCB               = NSButton(checkboxWithTitle: "Kick users", target: nil, action: nil)
+    private let canBanCB                = NSButton(checkboxWithTitle: "Ban users", target: nil, action: nil)
+    private let canMoveUsersCB          = NSButton(checkboxWithTitle: "Move users between channels", target: nil, action: nil)
+    private let canAdminAccountsCB      = NSButton(checkboxWithTitle: "Manage accounts", target: nil, action: nil)
+    private let isAdminCB               = NSButton(checkboxWithTitle: "Full administrator", target: nil, action: nil)
+
+    init(nickname: String, current: Permissions) {
+        self.targetNickname = nickname
+        self.initial = current
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 340, height: 250))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        buildUI()
+        applyInitial()
+    }
+
+    private func buildUI() {
+        let titleLabel = NSTextField(labelWithString: "Permissions — \(targetNickname)")
+        titleLabel.font = .boldSystemFont(ofSize: 13)
+
+        let checkboxes = [canCreateTempChannelCB, canKickCB, canBanCB,
+                          canMoveUsersCB, canAdminAccountsCB, isAdminCB]
+        for cb in checkboxes {
+            cb.setAccessibilityLabel(cb.title)
+        }
+
+        let cancelButton = NSButton(title: "Cancel", target: self, action: #selector(cancelClicked))
+        cancelButton.bezelStyle = .rounded
+
+        let saveButton = NSButton(title: "Save", target: self, action: #selector(saveClicked))
+        saveButton.bezelStyle = .rounded; saveButton.keyEquivalent = "\r"
+        saveButton.setAccessibilityLabel("Save permissions for \(targetNickname)")
+
+        let buttonRow = NSStackView(views: [NSView(), cancelButton, saveButton])
+        buttonRow.orientation = .horizontal; buttonRow.spacing = 8
+
+        var views: [NSView] = [titleLabel]
+        views.append(contentsOf: checkboxes)
+        views.append(buttonRow)
+
+        let stack = NSStackView(views: views)
+        stack.orientation = .vertical; stack.spacing = 8
+        stack.alignment = .leading
+        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),
+        ])
+    }
+
+    private func applyInitial() {
+        canCreateTempChannelCB.state = initial.canCreateTempChannel ? .on : .off
+        canKickCB.state             = initial.canKick              ? .on : .off
+        canBanCB.state              = initial.canBan               ? .on : .off
+        canMoveUsersCB.state        = initial.canMoveUsers         ? .on : .off
+        canAdminAccountsCB.state    = initial.canAdminAccounts     ? .on : .off
+        isAdminCB.state             = initial.isAdmin              ? .on : .off
+    }
+
+    @objc private func saveClicked() {
+        let perms = Permissions(
+            canCreateTempChannel: canCreateTempChannelCB.state == .on,
+            canKick:              canKickCB.state              == .on,
+            canBan:               canBanCB.state               == .on,
+            canMoveUsers:         canMoveUsersCB.state         == .on,
+            canAdminAccounts:     canAdminAccountsCB.state     == .on,
+            isAdmin:              isAdminCB.state              == .on
+        )
+        dismiss(nil)
+        onComplete?(perms)
+    }
+
+    @objc private func cancelClicked() {
+        dismiss(nil)
+        onComplete?(nil)
+    }
+}