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/PasswordPromptSheet.swift

@@ -0,0 +1,76 @@
+import AppKit
+
+final class PasswordPromptSheet: NSViewController {
+
+    var onComplete: ((String?) -> Void)?
+
+    private let prompt: String
+    private let passwordField = NSSecureTextField()
+
+    init(prompt: String) {
+        self.prompt = prompt
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 320, height: 110))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        buildUI()
+    }
+
+    private func buildUI() {
+        let promptLabel = NSTextField(labelWithString: prompt)
+        promptLabel.lineBreakMode = .byWordWrapping
+        promptLabel.setAccessibilityLabel(prompt)
+
+        passwordField.placeholderString = "password"
+        passwordField.setAccessibilityLabel("Password")
+        passwordField.target = self; passwordField.action = #selector(okClicked)
+
+        let cancelButton = NSButton(title: "Cancel", target: self, action: #selector(cancelClicked))
+        cancelButton.bezelStyle = .rounded
+        cancelButton.setAccessibilityLabel("Cancel")
+
+        let okButton = NSButton(title: "OK", target: self, action: #selector(okClicked))
+        okButton.bezelStyle = .rounded
+        okButton.keyEquivalent = "\r"
+        okButton.setAccessibilityLabel("Submit password")
+
+        let buttonRow = NSStackView(views: [NSView(), cancelButton, okButton])
+        buttonRow.orientation = .horizontal; buttonRow.spacing = 8
+
+        let stack = NSStackView(views: [promptLabel, passwordField, 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),
+        ])
+    }
+
+    override func viewDidAppear() {
+        super.viewDidAppear()
+        passwordField.becomeFirstResponder()
+    }
+
+    @objc private func okClicked() {
+        let pw = passwordField.stringValue
+        dismiss(nil)
+        onComplete?(pw)
+    }
+
+    @objc private func cancelClicked() {
+        dismiss(nil)
+        onComplete?(nil)
+    }
+}