fix: placement cells missing width/height and incorrect positioning

- Add width/height (30px) to .placement-cell CSS so cells are visible
- Account for CSS grid gap (1px) in placement cell positioning
- Set width/height dynamically in JS for consistency

src/blokus_ui/static/style.css

@@ -205,6 +205,8 @@ h3 {
 
 .placement-cell {
     position: absolute;
+    width: 30px;
+    height: 30px;
     background: rgba(255, 204, 0, 0.25);
     border: 1px dashed rgba(255, 204, 0, 0.7);
     pointer-events: auto;