fix: promotion dialog only fires for genuine pawn promotions

The "Promote pawn" dialog popped for any pawn "moved" toward the last
rank: the commit paths checked piece type + destination rank but never
the pawn's SOURCE rank. With the phantom layer now filling ranks 7-8
with tappable phantom pieces, tapping one (which falls through to the
real-move handler) while a real pawn was armed triggered the dialog for
a move no pawn could make — and for any phantom type, not just pawns.

Root cause: incomplete promotion detection, duplicated in Game.svelte
`onCommit` and the server's `isPromotionRequired`. Replaced with one
shared `isPromotionMove(piece, from, to)` — pawn, from the rank adjacent
to promotion, to the promotion rank, at most one file over — used by
both. 7 unit tests in packages/shared.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shared/src/index.ts

@@ -3,3 +3,4 @@ export * from './moderator.js';
 export * from './protocol.js';
 export * from './geometric.js';
 export * from './phantoms.js';
+export * from './promotion.js';

packages/shared/src/promotion.ts

@@ -0,0 +1,23 @@
+import { fileIndex, type Piece, type Square } from './types.js';
+
+/**
+ * True iff moving `piece` from→to is a pawn promotion.
+ *
+ * A promotion is a pawn advancing from the rank adjacent to its promotion rank
+ * onto the promotion rank (7→8 for White, 2→1 for Black), at most one file
+ * over — a straight push or a diagonal capture.
+ *
+ * Checking only the piece type and the destination rank — as the move-commit
+ * paths previously did — wrongly flags e.g. a pawn on the 2nd rank "moved" to
+ * the 8th, popping the promotion dialog for a move no pawn could ever make.
+ */
+export function isPromotionMove(
+  piece: Piece | undefined,
+  from: Square,
+  to: Square,
+): boolean {
+  if (!piece || piece.type !== 'p') return false;
+  if (Math.abs(fileIndex(from) - fileIndex(to)) > 1) return false;
+  if (piece.color === 'w') return from[1] === '7' && to[1] === '8';
+  return from[1] === '2' && to[1] === '1';
+}