Stage 7 UI polish: fix ad-marquee end, drop-time zoom, focus centring, sticky rack selection

- AdBanner: move the side inset onto the scrolling track so the long message
  scrolls to its very end; pin body text-size-adjust:100% so iOS/Safari stops
  inflating the long marquee text.
- Game: do not zoom on drag start (the player may change their mind) — zoom and
  centre happen on drop, in attemptPlace; a stray tap on an occupied cell no
  longer cancels the rack selection (wait for an empty cell).
- Board: centre the focus cell after the zoom width transition finishes (was
  clamping to top-left mid-transition); compute the cell from the rendered
  scrollWidth.

ui/src/game/Board.svelte

@@ -47,12 +47,29 @@
   $effect(() => {
     const vp = viewport;
     if (!vp || !zoomed || !focus) return;
-    const cell = (vp.clientWidth * Z) / 15;
-    vp.scrollTo({
-      left: (focus.col + 0.5) * cell - vp.clientWidth / 2,
-      top: (focus.row + 0.5) * cell - vp.clientHeight / 2,
-      behavior: 'smooth',
-    });
+    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();
   });
 
   // Double-tap toggles zoom (pinch was dropped — it conflicts with native scroll).