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/Board.svelte

@@ -4,6 +4,7 @@
   import type { Premium } from '../lib/premiums';
   import { valueForLetter } from '../lib/alphabet';
   import type { Variant } from '../lib/model';
+  import { usesStarBlank, BLANK_STAR } from '../lib/variants';
   import { bonusLabel, type BoardLabelMode } from '../lib/boardlabels';
   import type { Locale } from '../lib/i18n/catalog';
 
@@ -254,7 +255,11 @@
           >
             {#if letter}
               <span class="letter">{letter}</span>
-              {#if !blank}<span class="val">{valueForLetter(variant, letter)}</span>{/if}
+              {#if !blank}
+                <span class="val">{valueForLetter(variant, letter)}</span>
+              {:else if usesStarBlank(variant)}
+                <span class="val blankmark">{BLANK_STAR}</span>
+              {/if}
             {:else if r === centre.row && c === centre.col}
               <span class="star">★</span>
             {:else if bl?.kind === 'single'}
@@ -410,6 +415,10 @@
     font-size: 2.4cqw;
     font-weight: 600;
   }
+  /* A placed Erudit blank ("звёздочка") shows its star where the (absent) point value sits. */
+  .blankmark {
+    font-size: 3.2cqw;
+  }
   .star {
     position: absolute;
     inset: 0;