fix(refboard): dot grid behind canvas, paste at native size

- Make PixiJS canvas background transparent so SVG dot grid shows through
- Move dot grid SVG before canvas in DOM with zIndex 0 (behind images)
- Set wrapper div background to #1e1e1e
- Remove 600px cap on pasted images — preserve native dimensions

frontend/src/canvas/PixiCanvas.tsx

@@ -119,7 +119,7 @@ const PixiCanvas = forwardRef<PixiCanvasHandle, PixiCanvasProps>(
 
       (async () => {
         await app.init({
-          background: '#1e1e1e',
+          backgroundAlpha: 0,
           resizeTo: container,
           antialias: true,
           autoDensity: true,