feat(ui): Erudit blank tiles carry the star (✻) mark

The Erudit variant's blank is the "звёздочка", so render it with a star.
An empty rack blank (and its drag ghost) shows ✻ centred; a placed blank
keeps its designated letter and carries ✻ where the (absent) point value
sits — on the board and in the Stats best-move tiles. The Scrabble variants
are unchanged. Gated by usesStarBlank() in lib/variants.ts.

ui/src/game/Game.svelte

@@ -17,7 +17,7 @@
   import { badgeKind } from '../lib/unread';
   import { historyGrid } from '../lib/history';
   import { centre, premiumGrid } from '../lib/premiums';
-  import { variantNameKey } from '../lib/variants';
+  import { variantNameKey, usesStarBlank, BLANK_STAR } from '../lib/variants';
   import { alphabetLetters, hasAlphabet } from '../lib/alphabet';
   import { hintsLeft } from '../lib/hints';
   import { shareOrDownloadGcg } from '../lib/share';
@@ -1293,7 +1293,7 @@
 
 {#if drag}
   <div class="ghost" class:touch={drag.touch} style="left:{drag.x}px; top:{drag.y}px">
-    <span>{drag.blank ? '' : drag.letter}</span>
+    <span>{drag.blank ? (usesStarBlank(variant) ? BLANK_STAR : '') : drag.letter}</span>
   </div>
 {/if}