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/Board.svelte

@@ -105,24 +105,27 @@
     prevZoom = on;
     prevRecenter = rc;
     if (landscape) {
-      // Height-driven board in a wide viewport: pan so the focused cell is centred. Interpolate
+      // A wide viewport overflows VERTICALLY as soon as the square board grows past its height, but
-      // BOTH scroll axes together by TIME over the board's grow/shrink transition; the browser
+      // HORIZONTALLY only once the board grows past the (much larger) width — so a per-axis scroll
-      // clamps each to the current scrollable range as the board resizes. The earlier per-frame
+      // moves the vertical axis early and the horizontal axis late (the browser pins scrollLeft to 0
-      // clamp-to-max reached the wide axis (large clientWidth) before the tall one, which the owner
+      // until the board is wider than the viewport): the board positioning "in two steps". Drive
-      // saw as the board positioning "in two steps" — a single time-eased motion moves the axes in
+      // BOTH axes by one progress q — how far the board has grown past the viewport width, the point
-      // sync. Time-based (not a scrollWidth-progress tween like portrait's) so it also settles on
+      // at which a horizontal pan first becomes possible. Until then q is 0 and the board simply
-      // zoom-out, where the board shrinks below the viewport and stops overflowing.
+      // zooms centred; past it both axes pan together in one diagonal motion. The full-zoom focus
+      // target is finalSL/ST on zoom-in and the current position on zoom-out (where final is 0), so
+      // q running 1→0 unwinds the scroll before the board shrinks back below the viewport.
       if (toggled || (recentered && on && f)) {
-        const startSL = vp.scrollLeft;
+        const scaler = vp.firstElementChild as HTMLElement | null;
-        const startST = vp.scrollTop;
+        const targetSL = on ? finalSL : vp.scrollLeft;
-        const t0 = performance.now();
+        const targetST = on ? finalST : vp.scrollTop;
-        const dur = 250; // ~the .scaler.land width/height transition (0.25s ease)
+        const span = Math.max(1, fullSide - clientW);
-        let raf = requestAnimationFrame(function tick(now: number) {
+        let raf = requestAnimationFrame(function tick() {
-          const p = Math.min(1, (now - t0) / dur);
+          const side = scaler ? scaler.getBoundingClientRect().width : fullSide;
-          const e = 1 - Math.pow(1 - p, 3); // easeOutCubic, close to the CSS ease curve
+          const q = Math.max(0, Math.min(1, (side - clientW) / span));
-          vp.scrollLeft = startSL + (finalSL - startSL) * e;
+          vp.scrollLeft = targetSL * q;
-          vp.scrollTop = startST + (finalST - startST) * e;
+          vp.scrollTop = targetST * q;
-          if (p < 1) raf = requestAnimationFrame(tick);
+          const settled = on ? side >= fullSide - 1 : side <= fitSide + 1;
+          if (!settled) raf = requestAnimationFrame(tick);
         });
         return () => cancelAnimationFrame(raf);
       }
@@ -356,6 +359,9 @@
     /* Clamp the pan at the board edge: kill the native rubber-band/overscroll so the zoomed
        board cannot be dragged past its content into empty space — it just stops at the edge. */
     overscroll-behavior: none;
+    /* The zoom effect drives scrollLeft/Top itself while the board grows/shrinks; turn off the
+       browser's scroll-anchoring so it does not fight those writes mid-transition. */
+    overflow-anchor: none;
   }
   /* The query container is the (zoom-scaled) board, so cqw labels scale WITH the board
      — a magnifying-glass zoom. */

ui/src/game/Rack.svelte

@@ -121,9 +121,12 @@
     container-type: inline-size;
   }
   .rack.landscape .tile {
-    flex: 1 1 0;
+    /* Fixed tile size (1/7 of the fixed-width rack, accounting for the six 5px gaps), NOT flex-grow:
-    width: auto;
+       so placing a tile leaves the remaining ones put instead of growing them to refill the row —
-    min-width: 0;
+       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 {