- 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