Extend managed macOS client toward feature parity

clients/apple/dotnet/VoiceCat.Mac/MacKeychainPasswordStore.cs

@@ -19,6 +19,17 @@ internal sealed class MacKeychainPasswordStore
         finally { CryptographicOperations.ZeroMemory(encoded); }
     }
 
+    internal string? LoadLegacy(string? keychainTag)
+    {
+        if (string.IsNullOrWhiteSpace(keychainTag)) return null;
+        using var query = new SecRecord(SecKind.GenericPassword) { Service = "cat.voice.VoiceCatMac", Account = keychainTag };
+        using SecRecord? result = SecKeyChain.QueryAsRecord(query, out SecStatusCode status);
+        if (status != SecStatusCode.Success || result?.ValueData is not { } data) return null;
+        byte[] encoded = data.ToArray();
+        try { return Encoding.UTF8.GetString(encoded); }
+        finally { CryptographicOperations.ZeroMemory(encoded); }
+    }
+
     internal void Save(Guid profileId, string password)
     {
         ArgumentException.ThrowIfNullOrEmpty(password);