fix(ui): landscape zoom two-step (sync both axes) + fixed rack tile size

Follow-up to the contour review of the landscape fixes:

- Zoom still positioned "in two steps": a wide viewport overflows vertically as soon as
  the square board grows past its height, but horizontally only once it grows past the
  (much wider) width — so the browser pins scrollLeft to 0 while the vertical axis
  already pans (no time/ease tween fixes it; the horizontal scroll range physically is
  not there yet). Found by instrumenting the scroll trajectory. Now drive both axes by
  one progress = how far the board has grown past the viewport width (when a horizontal
  pan first becomes possible): until then the board just zooms centred, past it both
  axes pan together in one diagonal motion. Also disable scroll-anchoring so the browser
  stops fighting the programmatic scroll mid-transition. Re-verified: both axes now
  start and move together.
- Rack tiles resized when a tile was placed: landscape used flex-grow, so removing a
  tile regrew the rest. Give them a fixed size (1/7 of the fixed-width rack), like the
  portrait rack, so placing a tile leaves the rest put.

ui/src/game/Rack.svelte

@@ -121,9 +121,12 @@
     container-type: inline-size;
   }
   .rack.landscape .tile {
-    flex: 1 1 0;
-    width: auto;
-    min-width: 0;
+    /* Fixed tile size (1/7 of the fixed-width rack, accounting for the six 5px gaps), NOT flex-grow:
+       so placing a tile leaves the remaining ones put instead of growing them to refill the row —
+       matching the portrait rack. The constant rack width also keeps the cqw glyphs above steady as
+       tiles leave. */
+    flex: 0 0 auto;
+    width: calc((100% - 6 * 5px) / 7);
     max-width: 46px;
   }
   .rack.landscape .letter {