fix(ui): pan the zoomed board via window listeners, not pointer capture

The mouse drag-to-pan of the zoomed board captured the pointer on the scroll container
(vp.setPointerCapture) and wrote that same element's scrollLeft/Top. On an older
Chromium — the Chrome 74 Android 10 System WebView — capturing the pointer on the very
element being scrolled drops or displaces the capture as the scroll offset changes
mid-drag, so the board juddered under the cursor and would not pan (modern engines
cope).

Track the drag with window pointermove/up/cancel listeners, added on pointerdown and
removed on release, instead of capturing — the same "keep receiving moves past the
element edge" without holding the scroll container. The path is mouse-only (touch
scrolls the overflow viewport natively) and its behaviour on modern engines is
unchanged; svelte-check and the mock e2e (incl. the zoom specs) are green.

Speculative, not locally reproducible (needs Chrome 74): pending the owner's on-device
confirmation. The unrelated auto-zoom-on-placement gap on the same engine is left as-is
(minor).

ui/src/game/Board.svelte

@@ -228,16 +228,11 @@
     let sy = 0;
     let sl = 0;
     let st = 0;
-    const onDown = (e: PointerEvent) => {
-      if (e.pointerType === 'touch' || e.button !== 0 || !zoomed) return;
-      if ((e.target as HTMLElement | null)?.closest('.cell.pending')) return;
-      armed = true;
-      panning = false;
-      sx = e.clientX;
-      sy = e.clientY;
-      sl = vp.scrollLeft;
-      st = vp.scrollTop;
-    };
+    // Track the drag on window rather than vp.setPointerCapture. Capturing the pointer on the very
+    // element being scrolled makes an older Chromium — the Chrome 74 Android 10 System WebView —
+    // drop or displace the capture as scrollLeft/Top are written mid-drag, so the board judders and
+    // will not pan. Window listeners give the same "keep receiving moves past the element edge"
+    // without capturing the scroll container; modern engines behave identically.
     const onMove = (e: PointerEvent) => {
       if (!armed) return;
       const dx = e.clientX - sx;
@@ -245,16 +240,17 @@
       if (!panning) {
         if (Math.hypot(dx, dy) < 4) return; // a small move is still a click, not a pan
         panning = true;
-        vp.setPointerCapture(e.pointerId);
       }
       vp.scrollLeft = sl - dx;
       vp.scrollTop = st - dy;
     };
-    const onUp = (e: PointerEvent) => {
+    const onUp = () => {
+      window.removeEventListener('pointermove', onMove);
+      window.removeEventListener('pointerup', onUp);
+      window.removeEventListener('pointercancel', onUp);
       armed = false;
       if (!panning) return;
       panning = false;
-      if (vp.hasPointerCapture(e.pointerId)) vp.releasePointerCapture(e.pointerId);
       // Swallow the click the drag would otherwise produce (capture phase, before it reaches the
       // cell); self-remove on that click, and clean up on the next tick if none fired.
       const swallow = (ev: Event) => {
@@ -264,15 +260,25 @@
       vp.addEventListener('click', swallow, true);
       setTimeout(() => vp.removeEventListener('click', swallow, true), 0);
     };
+    const onDown = (e: PointerEvent) => {
+      if (e.pointerType === 'touch' || e.button !== 0 || !zoomed) return;
+      if ((e.target as HTMLElement | null)?.closest('.cell.pending')) return;
+      armed = true;
+      panning = false;
+      sx = e.clientX;
+      sy = e.clientY;
+      sl = vp.scrollLeft;
+      st = vp.scrollTop;
+      window.addEventListener('pointermove', onMove);
+      window.addEventListener('pointerup', onUp);
+      window.addEventListener('pointercancel', onUp);
+    };
     vp.addEventListener('pointerdown', onDown);
-    vp.addEventListener('pointermove', onMove);
-    vp.addEventListener('pointerup', onUp);
-    vp.addEventListener('pointercancel', onUp);
     return () => {
       vp.removeEventListener('pointerdown', onDown);
-      vp.removeEventListener('pointermove', onMove);
-      vp.removeEventListener('pointerup', onUp);
-      vp.removeEventListener('pointercancel', onUp);
+      window.removeEventListener('pointermove', onMove);
+      window.removeEventListener('pointerup', onUp);
+      window.removeEventListener('pointercancel', onUp);
     };
   });