fix(ui): raise the rack blank star to centre on the letters

Top-aligning it still read a touch low; move the empty-blank star up
(top 8% -> 0.5%) so its ink centres against the rack letters' block,
matching the board tile's centred mark. Size unchanged.

ui/src/game/Rack.svelte

@@ -139,10 +139,11 @@
     font-weight: 600;
   }
   /* Erudit's blank ("звёздочка") shows its star horizontally centred on the otherwise empty
-     tile face, top-aligned with the neighbouring letters (slightly larger so it reads). */
+     tile face; the small top offset centres its ink against the neighbouring letters' block
+     (it is slightly larger than them so it reads). */
   .star {
     position: absolute;
-    top: 8%;
+    top: 0.5%;
     left: 0;
     right: 0;
     text-align: center;