fix(web): explicit top/left on .slot and .recycle-label

Without top:0;left:0, Firefox and other non-Chrome engines place
absolute elements at the content edge (padding offset = 20px) before
the JS transform is applied, shifting slots 20px below/right of cards.
Cards already had explicit top:0;left:0; slots now match.

.recycle-label also had top:50%;left:50% which combined with the JS
inline transform would place the ↺ symbol halfway across the board.
Changed to top:0;left:0 so JS transform is the sole position source.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_server/web/game.css

@@ -106,6 +106,7 @@ main {
 /* Empty-pile slot markers */
 .slot {
     position: absolute;
+    top: 0; left: 0;   /* explicit insets — without these some browsers offset by padding */
     width: var(--card-w);
     height: var(--card-h);
     border: 2px dashed rgba(255,255,255,0.15);
@@ -187,11 +188,10 @@ main {
     box-shadow: 0 0 0 2px var(--accent), 0 4px 12px rgba(0,0,0,0.5);
 }
 
-/* Recycle indicator on empty stock */
+/* Recycle indicator on empty stock — JS sets transform to position it */
 .recycle-label {
     position: absolute;
-    top: 50%; left: 50%;
+    top: 0; left: 0;
-    transform: translate(-50%, -50%);
     font-size: 26px;
     color: rgba(255,255,255,0.3);
     pointer-events: none;