chore(client): suppress phantom-span a11y warning with documented svelte-ignore

Phantom pieces are a pointer-only overlay; adding role/tabindex would create
a focusable element with no keyboard equivalent (worse a11y, not better).
The real game remains fully keyboard-operable via the square <button>.

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

packages/client/src/lib/Board.svelte

@@ -122,6 +122,10 @@
           <span class="piece piece-{piece.color}">{pieceGlyph(piece)}</span>
         {/if}
         {#if ph && !piece}
+          <!-- Phantom pieces are a pointer-only private annotation overlay;
+               there is no keyboard drag interaction. The real game stays fully
+               keyboard-operable via the square button below. -->
+          <!-- svelte-ignore a11y_no_static_element_interactions -->
           <span
             class="phantom phantom-{ph.color}"
             class:dragging={sq === dragOrigin}