fix(client): handle pointercancel and make drag-start idempotent

Add onCancel handler for browser/OS gesture takeover (scroll, palm
rejection) that previously leaked window listeners and left drag stuck.
Extract detach() helper called by onUp, onCancel, and start() for
idempotency.

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

packages/client/src/lib/stores/phantom-drag.svelte.ts

@@ -27,6 +27,12 @@ function makeDrag() {
   let suppressClickOn: Square | null = null;
   const THRESHOLD = 6;
 
+  function detach() {
+    window.removeEventListener('pointermove', onMove);
+    window.removeEventListener('pointerup', onUp);
+    window.removeEventListener('pointercancel', onCancel);
+  }
+
   function onMove(e: PointerEvent) {
     if (!state.active) return;
     state.x = e.clientX;
@@ -36,15 +42,23 @@ function makeDrag() {
     }
   }
 
+  // pointercancel fires instead of pointerup when the browser/OS takes over
+  // the gesture (common on touch). Abort the drag: clean up, drop nothing.
+  function onCancel() {
+    detach();
+    state.active = null;
+    state.moved = false;
+  }
+
   function onUp(e: PointerEvent) {
-    window.removeEventListener('pointermove', onMove);
-    window.removeEventListener('pointerup', onUp);
+    detach();
     const src = state.active;
     const wasDrag = state.moved;
     state.active = null;
     state.moved = false;
     if (!src || !wasDrag) return; // a tap — the board click handler deals with it
 
+    // elementFromPoint returns null off-viewport — treated as an off-board drop.
     const el = document.elementFromPoint(e.clientX, e.clientY);
     const sqEl = el?.closest('[data-square]') as HTMLElement | null;
     const target = sqEl?.dataset.square as Square | undefined;
@@ -61,6 +75,7 @@ function makeDrag() {
   }
 
   function start(src: DragSource, e: PointerEvent) {
+    detach(); // idempotency — drop any listeners from an unfinished prior drag
     suppressClickOn = null;
     state.active = src;
     state.x = startX = e.clientX;
@@ -68,6 +83,7 @@ function makeDrag() {
     state.moved = false;
     window.addEventListener('pointermove', onMove);
     window.addEventListener('pointerup', onUp);
+    window.addEventListener('pointercancel', onCancel);
   }
 
   /** The board calls this first in its square-click handler. */