fix(ui): a board pinch-zoom no longer triggers swipe-back

The edge swipe-back armed on the first finger and fired on release even when a
second finger had joined (a pinch-zoom — the board is not .zoomed yet at the first
touch, so the hit-test could not skip it). Track the live pointer count in the
capture phase and cancel the swipe the moment a second pointer joins; the back
navigation fires only for a lone finger. Synthetic-PointerEvent e2e covers both.

ui/src/components/Screen.svelte

@@ -42,22 +42,53 @@
   // a zoomed-in board (it pans), and text inputs.
   const EDGE_FRACTION = 0.5;
   const SWIPE_SKIP = '[data-rack], .cell.pending, .viewport.zoomed, input, textarea, select';
+  // A pinch-zoom on the board is two fingers; its second finger lands after the gesture began
+  // (the board is not yet .zoomed at the first touch, so the hit-test above can't skip it), and
+  // releasing it would otherwise read as a horizontal back-swipe. Track the live pointer count
+  // (in the capture phase, so we see every touch even if the board stops propagation) and cancel
+  // the swipe the moment a second finger joins. The swipe fires only for a lone finger.
   $effect(() => {
+    let startX = 0;
+    let startY = 0;
+    let active = 0; // pointers currently down
+    let tracking = false; // a candidate single-finger edge swipe is in progress
+    let pinched = false; // a second finger joined — not a back-swipe
+
     function onDown(e: PointerEvent) {
+      active++;
+      if (active > 1) {
+        pinched = true;
+        return;
+      }
+      tracking = false;
+      pinched = false;
       if (!back || e.pointerType === 'mouse' || e.clientX > window.innerWidth * EDGE_FRACTION) return;
       if (document.elementFromPoint(e.clientX, e.clientY)?.closest(SWIPE_SKIP)) return;
-      const x0 = e.clientX;
-      const y0 = e.clientY;
-      const onUp = (ev: PointerEvent) => {
-        window.removeEventListener('pointerup', onUp, true);
-        const dx = ev.clientX - x0;
-        const dy = ev.clientY - y0;
-        if (back && dx > 64 && Math.abs(dx) > Math.abs(dy) * 1.4) navigate(back);
-      };
-      window.addEventListener('pointerup', onUp, true);
+      startX = e.clientX;
+      startY = e.clientY;
+      tracking = true;
     }
+
+    function onUp(e: PointerEvent) {
+      if (active > 0) active--;
+      if (active > 0) return; // wait until every finger has lifted
+      if (tracking && !pinched && back) {
+        const dx = e.clientX - startX;
+        const dy = e.clientY - startY;
+        if (dx > 64 && Math.abs(dx) > Math.abs(dy) * 1.4) navigate(back);
+      }
+      tracking = false;
+      pinched = false;
+    }
+
     window.addEventListener('pointerdown', onDown, true);
-    return () => window.removeEventListener('pointerdown', onDown, true);
+    window.addEventListener('pointerup', onUp, true);
+    window.addEventListener('pointercancel', onUp, true);
+    return () => {
+      window.removeEventListener('pointerdown', onDown, true);
+      window.removeEventListener('pointerup', onUp, true);
+      window.removeEventListener('pointercancel', onUp, true);
+    };
   });
 </script>