refactor: add VisualAsset.shallowCopy() to eliminate 9-field manual entity copy in previewTransform

src/main/java/dev/kruhlmann/imgfloat/model/db/imgfloat/VisualAsset.java

@@ -64,6 +64,29 @@ public class VisualAsset {
         }
     }
 
+    /**
+     * Returns a detached, unmanaged copy of this entity suitable for preview/simulation use.
+     * The copy shares the same field values but is not associated with any JPA persistence context.
+     */
+    public VisualAsset shallowCopy() {
+        VisualAsset copy = new VisualAsset();
+        copy.id = this.id;
+        copy.name = this.name;
+        copy.preview = this.preview;
+        copy.x = this.x;
+        copy.y = this.y;
+        copy.width = this.width;
+        copy.height = this.height;
+        copy.rotation = this.rotation;
+        copy.speed = this.speed;
+        copy.muted = this.muted;
+        copy.mediaType = this.mediaType;
+        copy.originalMediaType = this.originalMediaType;
+        copy.audioVolume = this.audioVolume;
+        copy.hidden = this.hidden;
+        return copy;
+    }
+
     public String getId() {
         return id;
     }

src/main/java/dev/kruhlmann/imgfloat/service/ChannelDirectoryService.java

@@ -839,17 +839,7 @@ public class ChannelDirectoryService {
             visual.getAudioVolume()
         );
 
-        VisualAsset previewState = new VisualAsset();
+        VisualAsset previewState = visual.shallowCopy();
-        previewState.setId(visual.getId());
-        previewState.setName(visual.getName());
-        previewState.setX(visual.getX());
-        previewState.setY(visual.getY());
-        previewState.setWidth(visual.getWidth());
-        previewState.setHeight(visual.getHeight());
-        previewState.setRotation(visual.getRotation());
-        previewState.setSpeed(visual.getSpeed());
-        previewState.setMuted(visual.isMuted());
-        previewState.setAudioVolume(visual.getAudioVolume());
 
         if (previewRequest.getX() != null) {
             previewState.setX(previewRequest.getX());