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/ServerListStore.swift

@@ -0,0 +1,69 @@
+import Foundation
+import Security
+
+enum ServerListStore {
+    static var appSupportURL: URL {
+        let url = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).first!
+        return url.appendingPathComponent("VoiceCat", isDirectory: true)
+    }
+
+    static var serversURL: URL {
+        appSupportURL.appendingPathComponent("servers.json")
+    }
+
+    static var tofuStorePath: String {
+        appSupportURL.appendingPathComponent("tofu_pins.txt").path
+    }
+
+    static func load() -> [SavedServer] {
+        try? FileManager.default.createDirectory(at: appSupportURL, withIntermediateDirectories: true)
+        guard let data = try? Data(contentsOf: serversURL),
+              let list = try? JSONDecoder().decode([SavedServer].self, from: data) else { return [] }
+        return list
+    }
+
+    static func save(_ servers: [SavedServer]) {
+        try? FileManager.default.createDirectory(at: appSupportURL, withIntermediateDirectories: true)
+        if let data = try? JSONEncoder().encode(servers) {
+            try? data.write(to: serversURL, options: .atomic)
+        }
+    }
+
+    // MARK: - Keychain
+
+    static func savePassword(_ password: String, tag: String) {
+        guard let data = password.data(using: .utf8) else { return }
+        deletePassword(tag: tag)
+        let query: [CFString: Any] = [
+            kSecClass: kSecClassGenericPassword,
+            kSecAttrService: "cat.voice.VoiceCatMac",
+            kSecAttrAccount: tag,
+            kSecValueData: data,
+        ]
+        SecItemAdd(query as CFDictionary, nil)
+    }
+
+    static func loadPassword(tag: String) -> String? {
+        let query: [CFString: Any] = [
+            kSecClass: kSecClassGenericPassword,
+            kSecAttrService: "cat.voice.VoiceCatMac",
+            kSecAttrAccount: tag,
+            kSecReturnData: true,
+            kSecMatchLimit: kSecMatchLimitOne,
+        ]
+        var result: AnyObject?
+        guard SecItemCopyMatching(query as CFDictionary, &result) == errSecSuccess,
+              let data = result as? Data,
+              let password = String(data: data, encoding: .utf8) else { return nil }
+        return password
+    }
+
+    static func deletePassword(tag: String) {
+        let query: [CFString: Any] = [
+            kSecClass: kSecClassGenericPassword,
+            kSecAttrService: "cat.voice.VoiceCatMac",
+            kSecAttrAccount: tag,
+        ]
+        SecItemDelete(query as CFDictionary)
+    }
+}