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/Game.svelte

@@ -132,7 +132,8 @@
       dragMoved = true;
       const slot = placement.rack[downInfo.index];
       drag = { letter: slot, blank: slot === BLANK, x: e.clientX, y: e.clientY };
-      if (isCoarse() && !zoomed) zoomed = true;
+      // No zoom on drag start: the player may still change their mind. The zoom
+      // (and centring) happens on drop, in attemptPlace.
     }
     if (drag) drag = { ...drag, x: e.clientX, y: e.clientY };
   }
@@ -169,6 +170,9 @@
       return;
     }
     if (selected != null) {
+      // A committed tile already sits here: keep the rack selection so a stray tap
+      // on an occupied cell doesn't cancel placement — wait for an empty cell.
+      if (board[row]?.[col]) return;
       attemptPlace(selected, row, col);
       selected = null;
     }