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/lib/variants.test.ts

@@ -5,6 +5,8 @@ import {
   availableVariants,
   supportsMultipleWordsToggle,
   multipleWordsForRequest,
+  usesStarBlank,
+  BLANK_STAR,
 } from './variants';
 
 describe('ALL_VARIANTS', () => {
@@ -53,3 +55,16 @@ describe('multipleWordsForRequest', () => {
     expect(multipleWordsForRequest('scrabble_en', true)).toBe(true);
   });
 });
+
+describe('usesStarBlank', () => {
+  it('marks the blank with a star for Erudit only', () => {
+    expect(usesStarBlank('erudit_ru')).toBe(true);
+    expect(usesStarBlank('scrabble_ru')).toBe(false);
+    expect(usesStarBlank('scrabble_en')).toBe(false);
+  });
+
+  it('BLANK_STAR is the heavy teardrop-spoked asterisk (U+273B)', () => {
+    expect(BLANK_STAR).toBe('✻');
+    expect(BLANK_STAR.codePointAt(0)).toBe(0x273b);
+  });
+});