Stage 7 polish: game rework + board zoom + tests (Parts D/E/F/I)

- Board: fixed-viewport transform-scale zoom (animated) with counter-scaled cqw labels, corner letters, bonus-label modes (boardlabels), contrasting grid lines
- Game: Screen shell + game tab-bar (Draw/Skip/Hint/Shuffle) via HoldConfirm popovers; MakeMove 🏁 + compact popup; rack collapses used slots; hint places tiles on board (placementFromHint) + no_hint_available toast; Scores:N replaces Hints; history slide-down (swipe/click, scroll-locked); check-word alphabet/length limit + in-memory cache + 5s throttle
- backend: no_hint_available result code split + test
- vitest: banner rotator + linkify, resultBadge, boardlabels, placementFromHint (29 tests); Playwright smoke updated; prod bundle ~74 KB gzip

ui/src/lib/placement.test.ts

@@ -4,6 +4,7 @@ import {
   direction,
   newPlacement,
   place,
+  placementFromHint,
   rackView,
   recallAt,
   reset,
@@ -62,3 +63,19 @@ describe('placement state machine', () => {
     expect(toSubmit(newPlacement(rack))).toBeNull();
   });
 });
+
+describe('placementFromHint', () => {
+  it('maps hint letters and blanks onto rack slots', () => {
+    const p = placementFromHint(
+      [
+        { row: 7, col: 7, letter: 'C', blank: false },
+        { row: 7, col: 8, letter: 'A', blank: false },
+        { row: 7, col: 9, letter: 'B', blank: true },
+      ],
+      ['C', 'A', BLANK, 'T'],
+    );
+    expect(p.pending).toHaveLength(3);
+    expect(p.pending[0]).toMatchObject({ rackIndex: 0, letter: 'C', blank: false });
+    expect(p.pending[2]).toMatchObject({ rackIndex: 2, letter: 'B', blank: true });
+  });
+});