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/lib/placement.test.ts

@@ -6,6 +6,7 @@ import {
   newPlacement,
   place,
   placementFromHint,
+  previewFromHint,
   rackView,
   recallAt,
   recallIndex,
@@ -105,6 +106,24 @@ describe('placementFromHint', () => {
   });
 });
 
+describe('previewFromHint', () => {
+  it('reuses the hint move as a legal preview, carrying score, words and direction', () => {
+    const preview = previewFromHint({
+      player: 0,
+      action: 'play',
+      dir: 'V',
+      mainRow: 7,
+      mainCol: 7,
+      tiles: [{ row: 7, col: 7, letter: 'C', blank: false }],
+      words: ['CAT', 'AN'],
+      count: 0,
+      score: 18,
+      total: 42,
+    });
+    expect(preview).toEqual({ legal: true, score: 18, words: ['CAT', 'AN'], dir: 'V' });
+  });
+});
+
 describe('reorderIndices', () => {
   it('lifts an element and drops it at the given slot among the others', () => {
     expect(reorderIndices(4, 0, 2)).toEqual([1, 2, 0, 3]);