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/components/AdBanner.svelte

@@ -56,7 +56,7 @@
   .ad {
     overflow: hidden;
     white-space: nowrap;
-    padding: 6px var(--pad);
+    padding: 6px 0;
     background: var(--surface-2);
     color: var(--text-muted);
     font-size: 0.85rem;
@@ -68,6 +68,9 @@
   }
   .track {
     display: inline-block;
+    /* The side inset lives on the track (not the clipping .ad) so the scroll distance
+       (scrollWidth - viewport.clientWidth) reaches the very end of a long message. */
+    padding: 0 var(--pad);
     will-change: transform;
   }
   .track :global(a) {