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/boardlabels.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { bonusLabel } from './boardlabels';
+
+describe('bonusLabel', () => {
+  it('none mode and plain squares have no label', () => {
+    expect(bonusLabel('none', 'TW', 'en')).toBeNull();
+    expect(bonusLabel('beginner', '', 'en')).toBeNull();
+  });
+
+  it('classic is a localized single tag', () => {
+    expect(bonusLabel('classic', 'TW', 'en')).toEqual({ kind: 'single', text: '3W' });
+    expect(bonusLabel('classic', 'DL', 'en')).toEqual({ kind: 'single', text: '2L' });
+    expect(bonusLabel('classic', 'TW', 'ru')).toEqual({ kind: 'single', text: '3С' });
+    expect(bonusLabel('classic', 'DL', 'ru')).toEqual({ kind: 'single', text: '2Б' });
+  });
+
+  it('beginner is a localized split label', () => {
+    expect(bonusLabel('beginner', 'TW', 'en')).toEqual({ kind: 'split', top: '3×', bottom: 'word' });
+    expect(bonusLabel('beginner', 'DL', 'en')).toEqual({ kind: 'split', top: '2×', bottom: 'letter' });
+    expect(bonusLabel('beginner', 'TL', 'ru')).toEqual({ kind: 'split', top: '3×', bottom: 'буква' });
+  });
+});