Fix garbage elimination bug and align board with header

- Change y-- instead of y++ when pushing piece up on garbage
- Add y < 0 check to properly detect piece pushed off top
- Remove margin-top from battle grid to align with header

Co-Authored-By: Claude Opus 4.6 <[email protected]>

public/css/style.css

@@ -197,7 +197,6 @@ button:active {
   display: flex;
   justify-content: center;
   align-items: center;
-  margin-top: 20px;
 }
 
 /* Main player board - centered and large */