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/Models/SavedServer.swift

@@ -0,0 +1,24 @@
+import Foundation
+
+enum AuthMode: String, Codable, CaseIterable {
+    case guest
+    case password
+}
+
+struct SavedServer: Codable, Identifiable {
+    var id: UUID = UUID()
+    var host: String
+    var port: UInt16
+    var authMode: AuthMode
+    var savedUsername: String?
+    var keychainTag: String?
+
+    var displayString: String {
+        switch authMode {
+        case .guest:
+            return "\(host):\(port) (Guest)"
+        case .password:
+            return "\(savedUsername ?? "")@\(host):\(port)"
+        }
+    }
+}