feat: implement hint system (Shift+H) with scoring penalties

- Add getHint() method to BoardManager with two-tier priority:
  1. Find incorrect user-entered cells → flash red
  2. Find empty cells → show correct digit green temporarily
- Add Shift+H key handler in useCursor (normal mode only)
- Visual feedback:
  - Red flash (hint-flash-error): incorrect cells, 1s duration, red bg + border
  - Green flash (hint-flash-correct): correct digit shown, 1s duration, green bg + border
  - Distinct from invalid-move (no background, 500ms)
- Score penalties based on difficulty:
  - Easy: 25 points per hint
  - Medium: 50 points per hint
  - Hard: 100 points per hint
- Track hints used in scoreboard display
- Correct digit appears temporarily (1s) then disappears (player still does the work)
- Update example app to show hints counter and Shift+H instruction

src/useGame.ts

@@ -16,11 +16,15 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
   
   const [keyLog, setKeyLog] = useState<{ key: string; timestamp: number }[]>([]);
   
-  const cursor = useCursor(gameStatusManager, containerRef, board);
+  const [hintsUsedRef] = useState({ current: 0 });
+  
+  const cursor = useCursor(gameStatusManager, containerRef, board, () => {
+    hintsUsedRef.current++;
+  });
   const cursorPosition = cursor.position();
   const cursorMode = cursor.mode();
   
-  const scoreManager = useScore({ board, gameStatusManager, keyLog, cursorMode, difficulty });
+  const scoreManager = useScore({ board, gameStatusManager, keyLog, cursorMode, difficulty, hintsUsed: hintsUsedRef.current });
   
   useCursorRender(containerRef, cursorPosition, cursorMode, gameStatus, boardRendered);