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/AddServerSheet.swift

@@ -0,0 +1,154 @@
+import AppKit
+
+final class AddServerSheet: NSViewController {
+
+    var onComplete: ((SavedServer?) -> Void)?
+
+    private var editing: SavedServer?
+
+    private let hostField = NSTextField()
+    private let portField: NSTextField = {
+        let f = NSTextField()
+        f.stringValue = "7878"
+        return f
+    }()
+    private let authPicker = NSPopUpButton()
+    private let usernameField = NSTextField()
+    private let passwordField = NSSecureTextField()
+    private let savePwCheckbox = NSButton(checkboxWithTitle: "Save password in Keychain", target: nil, action: nil)
+
+    init(editing: SavedServer?) {
+        self.editing = editing
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 380, height: 260))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        buildUI()
+        if let s = editing {
+            hostField.stringValue = s.host
+            portField.stringValue = "\(s.port)"
+            authPicker.selectItem(withTitle: s.authMode == .guest ? "Guest" : "Account")
+            usernameField.stringValue = s.savedUsername ?? ""
+            updateAuthVisibility()
+        }
+    }
+
+    private func buildUI() {
+        let title = NSTextField(labelWithString: editing == nil ? "Add Server" : "Edit Server")
+        title.font = .boldSystemFont(ofSize: 14)
+
+        let hostLabel = NSTextField(labelWithString: "Host:")
+        let portLabel = NSTextField(labelWithString: "Port:")
+        let authLabel = NSTextField(labelWithString: "Auth:")
+        let userLabel = NSTextField(labelWithString: "Username:")
+        let pwLabel = NSTextField(labelWithString: "Password:")
+
+        hostField.placeholderString = "hostname or IP"
+        hostField.setAccessibilityLabel("Server hostname or IP address")
+
+        portField.setAccessibilityLabel("Server port")
+
+        authPicker.addItem(withTitle: "Guest")
+        authPicker.addItem(withTitle: "Account")
+        authPicker.target = self; authPicker.action = #selector(authChanged)
+        authPicker.setAccessibilityLabel("Authentication mode")
+
+        usernameField.placeholderString = "username"
+        usernameField.setAccessibilityLabel("Username")
+
+        passwordField.placeholderString = "leave blank to enter at connect"
+        passwordField.setAccessibilityLabel("Password (optional — enter at connect time if blank)")
+
+        savePwCheckbox.setAccessibilityLabel("Save password in system Keychain")
+
+        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 server")
+
+        let grid = NSGridView(views: [
+            [hostLabel, hostField],
+            [portLabel, portField],
+            [authLabel, authPicker],
+            [userLabel, usernameField],
+            [pwLabel, passwordField],
+            [NSView(), savePwCheckbox],
+        ])
+        grid.rowSpacing = 8
+        grid.columnSpacing = 8
+        grid.column(at: 0).xPlacement = .trailing
+
+        let buttonRow = NSStackView(views: [NSView(), cancelButton, saveButton])
+        buttonRow.orientation = .horizontal; buttonRow.spacing = 8
+
+        let stack = NSStackView(views: [title, grid, buttonRow])
+        stack.orientation = .vertical
+        stack.spacing = 16
+        stack.edgeInsets = NSEdgeInsets(top: 20, left: 20, bottom: 20, right: 20)
+        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),
+        ])
+
+        updateAuthVisibility()
+    }
+
+    @objc private func authChanged() { updateAuthVisibility() }
+
+    private func updateAuthVisibility() {
+        let isAccount = authPicker.titleOfSelectedItem == "Account"
+        usernameField.isEnabled = isAccount
+        passwordField.isEnabled = isAccount
+        savePwCheckbox.isEnabled = isAccount
+    }
+
+    @objc private func saveClicked() {
+        let host = hostField.stringValue.trimmingCharacters(in: .whitespaces)
+        guard !host.isEmpty else {
+            hostField.becomeFirstResponder(); return
+        }
+        let portStr = portField.stringValue.trimmingCharacters(in: .whitespaces)
+        guard let port = UInt16(portStr), port > 0 else {
+            portField.becomeFirstResponder(); return
+        }
+        let isGuest = authPicker.titleOfSelectedItem == "Guest"
+        var server = editing ?? SavedServer(host: host, port: port,
+                                            authMode: isGuest ? .guest : .password)
+        server.host = host; server.port = port
+        server.authMode = isGuest ? .guest : .password
+        server.savedUsername = isGuest ? nil : usernameField.stringValue.trimmingCharacters(in: .whitespaces)
+
+        if !isGuest && savePwCheckbox.state == .on {
+            let pw = passwordField.stringValue
+            if !pw.isEmpty {
+                let tag = server.keychainTag ?? "voicecat.server.\(server.id.uuidString)"
+                server.keychainTag = tag
+                ServerListStore.savePassword(pw, tag: tag)
+            }
+        } else if isGuest {
+            if let tag = server.keychainTag { ServerListStore.deletePassword(tag: tag) }
+            server.keychainTag = nil
+        }
+
+        dismiss(nil)
+        onComplete?(server)
+    }
+
+    @objc private func cancelClicked() {
+        dismiss(nil)
+        onComplete?(nil)
+    }
+}