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.

docs/UI_DESIGN.md

@@ -109,7 +109,10 @@ dismisses as soon as the lobby is ready. The pure layout and timing live in `lib
 ## Tiles & board
 
 - **Tiles**: the letter sits in the **top-left** corner (offset a touch more than the
-  value), the point value bottom-right; blanks show no value.
+  value), the point value bottom-right; blanks show no value. In **Erudit** the blank is the
+  "звёздочка" (star) chip: an unplaced blank shows the star (`✻`, U+273B) centred on the rack
+  tile, and a placed blank carries it in the value corner; the Scrabble variants leave the
+  blank unmarked (`usesStarBlank` in `lib/variants.ts`).
 - **Board zoom** (`Board.svelte`): a two-state zoom (full 15×15 ↔ ~9 cells) by **growing
   the board's width** inside a fixed-size viewport (a real layout change → native scroll
   that works consistently across browsers; no `transform`, which broke scrolling