fix(opencv): correct contour type for findContours

Changed contours from cv.Mat to cv.MatVector to match OpenCV.js API
requirements. The findContours method expects a MatVector for contour
storage, not a single Mat object. This fixes a potential runtime error
when using OpenCV-based panel detection.

panel-detection/opencv.js

@@ -5,7 +5,7 @@ export async function detectPanelsOpenCV(imageData, cv) {
   const gray = new cv.Mat();
   const blurred = new cv.Mat();
   const edges = new cv.Mat();
-  const contours = new cv.Mat();
+  const contours = new cv.MatVector();
   const hierarchy = new cv.Mat();
 
   try {