perf(ui): reuse hint result as move preview, skip redundant evaluate

Taking a hint auto-placed the suggested tiles and then called recompute(),
which round-trips a debounced evaluate for that exact placement. The hint is
the engine's own top-ranked, fully scored legal move, so its move already
carries the score, words and direction an evaluate would return — the second
call was pure duplicate work and added a visible disabled->enabled flicker on
the submit button over slow links.

Seed the move preview directly from the hint move via previewFromHint and
cancel any pending evaluate timer so a stale one cannot clobber it. A later
manual edit re-arms recompute() as before, so rearranged tiles are re-evaluated
normally. Client-only; no backend, wire or schema change.

ui/src/game/Game.svelte

@@ -29,6 +29,7 @@
     newPlacement,
     place,
     placementFromHint,
+    previewFromHint,
     rackView,
     recallAt,
     recallToSlot,
@@ -751,7 +752,14 @@
         if (isCoarse()) zoomed = true;
         view = { ...view, hintsRemaining: h.hintsRemaining, walletBalance: h.walletBalance };
         syncWallet(h.walletBalance);
-        recompute();
+        // The hint is the engine's own top-ranked, fully scored legal move: reuse it as the
+        // preview instead of a redundant evaluate (same engine call, same placement). Cancel any
+        // pending evaluate so a stale one cannot clobber it; a later manual edit re-arms recompute().
+        if (previewTimer) {
+          clearTimeout(previewTimer);
+          previewTimer = null;
+        }
+        preview = previewFromHint(h.move);
       }
     } catch (e) {
       // The backend does not spend a hint when there is no move.