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/app.css

@@ -123,6 +123,9 @@ body {
   line-height: 1.4;
   -webkit-font-smoothing: antialiased;
   text-rendering: optimizeLegibility;
+  /* Stop iOS/Safari from auto-inflating text (e.g. the long marquee message). */
+  text-size-adjust: 100%;
+  -webkit-text-size-adjust: 100%;
   /* never let the page scroll/zoom out from under the board */
   overscroll-behavior: none;
   touch-action: manipulation;