Stage 7 UI polish: zoom-in magnifies into the focus cell (no top-left jump)

Drive the focus-centring with requestAnimationFrame across the ~0.25s width
transition instead of a single scrollTo after transitionend. The board now stays
locked on the placed cell as it grows, removing the visible 'centre top-left, then
correct' double motion.

ui/src/game/Board.svelte

@@ -43,33 +43,21 @@
   let viewport = $state<HTMLElement>();
 
   // Genuine layout zoom (the board grows; cqw labels stay constant), so native scroll
-  // works in every browser. Centre the focus cell when zoomed in.
+  // works in every browser. Keep the focus cell centred on every frame of the zoom-in
+  // (the board widens over ~0.25s) so it magnifies *into* that cell, rather than growing
+  // from the top-left corner and then jumping to centre once the transition ends.
   $effect(() => {
     const vp = viewport;
     if (!vp || !zoomed || !focus) return;
     const f = focus;
-    const scaler = vp.firstElementChild as HTMLElement | null;
-    const center = () => {
-      // Use the rendered scrollable width so the maths stays correct (gaps, padding).
-      const cell = vp.scrollWidth / 15;
-      vp.scrollTo({
-        left: (f.col + 0.5) * cell - vp.clientWidth / 2,
-        top: (f.row + 0.5) * cell - vp.clientHeight / 2,
-        behavior: 'smooth',
-      });
-    };
-    // When zoom has just turned on the board is still widening; centring now would
-    // clamp to the still-small scroll range and land top-left. Wait for the width
-    // transition to finish. If already zoomed (only the focus changed), centre at once.
-    if (scaler && scaler.clientWidth < vp.clientWidth * Z - 1) {
-      const onEnd = () => {
-        scaler.removeEventListener('transitionend', onEnd);
-        center();
-      };
-      scaler.addEventListener('transitionend', onEnd);
-      return () => scaler.removeEventListener('transitionend', onEnd);
-    }
-    center();
+    const start = performance.now();
+    let raf = requestAnimationFrame(function tick(now) {
+      const cell = vp.scrollWidth / 15; // grows frame by frame as the board widens
+      vp.scrollLeft = (f.col + 0.5) * cell - vp.clientWidth / 2;
+      vp.scrollTop = (f.row + 0.5) * cell - vp.clientHeight / 2;
+      if (now - start < 300) raf = requestAnimationFrame(tick);
+    });
+    return () => cancelAnimationFrame(raf);
   });
 
   // Double-tap toggles zoom (pinch was dropped — it conflicts with native scroll).