fix: ensure sprite preview has transparent background

Explicitly set backgroundColor: transparent to prevent
Pico CSS from overriding it.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/dev/app.tsx

@@ -76,6 +76,7 @@ const App = () => {
   const previewStyle = {
     width: `${width * scale}px`,
     height: `${height * scale}px`,
+    backgroundColor: "transparent",
     backgroundImage: `url('${spriteUrl}')`,
     backgroundSize: `${sheetWidth * scale}px ${sheetHeight * scale}px`,
     animation: `${keyframeId} ${totalDuration}ms steps(${frames}) infinite`,