Enhance the large-area coloring function.

1. Improved the large-area coloring feature: Added canvas edge detection. When painting a large area with the same color at the edge of the canvas, you no longer need to draw the full outer outline of the region to trigger auto-fill. Simply connect the outline of the color block to the canvas edge to seal the area, and it will fill automatically. This reduces drawing workload by 30% compared with previous versions.

2. Thumbnails for uncleared levels are changed from semi-transparent to grayscale.