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/server/src/commit.ts

@@ -1,6 +1,7 @@
 import type { Move } from 'chess.js';
 import {
   geometricMoves,
+  isPromotionMove,
   type Announcement,
   type Color,
   type Piece,
@@ -124,9 +125,6 @@ function chessJsLegalFrom(game: Game, from: Square): string[] {
 
 function isPromotionRequired(game: Game, from: Square, to: Square): boolean {
   const piece = game.chess.get(from);
-  if (!piece || piece.type !== 'p') return false;
-  const toRank = to[1];
-  if (piece.color === 'w' && toRank === '8') return true;
-  if (piece.color === 'b' && toRank === '1') return true;
-  return false;
+  if (!piece) return false;
+  return isPromotionMove({ color: piece.color, type: piece.type }, from, to);
 }