feat: complete legacy parity and modernize operator UI

src/MBN_STOCK_WEBVIEW.LegacyParityApp/MainWindow.Settings.cs

@@ -137,9 +137,11 @@ public sealed partial class MainWindow
                 },
                 _ => throw new ArgumentOutOfRangeException(nameof(kind))
             };
-            SaveOperatorSettings(
-                next,
-                $"{OperatorFolderLabel(kind)} 폴더를 저장했습니다. 앱을 다시 시작하면 적용됩니다.");
+            var savedMessage = kind == LegacyOperatorFolderKind.Design &&
+                validated.MissingOptionalExternalAssetCount > 0
+                    ? $"디자인 폴더를 저장했습니다. 외부 영상 {validated.MissingOptionalExternalAssetCount}개가 없어 관련 장면만 제한됩니다. 앱을 다시 시작하면 적용됩니다."
+                    : $"{OperatorFolderLabel(kind)} 폴더를 저장했습니다. 앱을 다시 시작하면 적용됩니다.";
+            SaveOperatorSettings(next, savedMessage);
         }
         catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
         {
@@ -251,7 +253,7 @@ public sealed partial class MainWindow
             LegacyOperatorSettingsFolderKind.Scene,
             _operatorSettings.SceneDirectory,
             "앱 기본 디자인 폴더",
-            "필수 장면 확인됨");
+            "활성 장면·기본 자산 확인됨");
         _resourceFolderSnapshot = CreateOperatorFolderSnapshot(
             LegacyOperatorSettingsFolderKind.Resource,
             _operatorSettings.ResourceDirectory,
@@ -290,8 +292,9 @@ public sealed partial class MainWindow
                 !LegacyOperatorFolderValidator.TryValidate(
                     kind,
                     defaultPath,
-                    out _,
-                    out _))
+                    out var defaultFolder,
+                    out _) ||
+                defaultFolder is null)
             {
                 return new LegacyOperatorFolderSnapshot(
                     defaultDisplay,
@@ -304,7 +307,7 @@ public sealed partial class MainWindow
                 defaultDisplay,
                 IsCustom: false,
                 IsValid: true,
-                validStatus);
+                FolderStatus(kind, defaultFolder, validStatus));
         }
 
         var validation = LegacyOperatorFolderValidator.Validate(kind, configuredPath);
@@ -320,7 +323,7 @@ public sealed partial class MainWindow
         var status = kind == LegacyOperatorSettingsFolderKind.Resource &&
             validation.Folder.DatabaseIniDetected
                 ? "UI·DB 설정 검증됨"
-                : validStatus;
+                : FolderStatus(kind, validation.Folder, validStatus);
         return new LegacyOperatorFolderSnapshot(
             validation.Folder.AbbreviatedPath,
             IsCustom: true,
@@ -328,6 +331,15 @@ public sealed partial class MainWindow
             status);
     }
 
+    private static string FolderStatus(
+        LegacyOperatorSettingsFolderKind kind,
+        LegacyOperatorValidatedFolder folder,
+        string validStatus) =>
+        kind == LegacyOperatorSettingsFolderKind.Scene &&
+        folder.MissingOptionalExternalAssetCount > 0
+            ? $"핵심 자산 확인됨 · 외부 영상 {folder.MissingOptionalExternalAssetCount}개 제외"
+            : validStatus;
+
     private LegacyOperatorSettingsSnapshot CreateOperatorSettingsSnapshot() => new(
         Volatile.Read(ref _operatorSettingsRevision),
         _designFolderSnapshot,