feat: implement scoring system and win condition (Phase 50)

- Add useTimer hook for elapsed time tracking
- Add useScore hook with comprehensive metrics:
  - Total keystrokes (all keys)
  - Edit keystrokes (only editing keys)
  - Optimal keystrokes (empty cells × 3)
  - Efficiency percentage (optimal/edit × 100)
  - Final score (70% efficiency + 30% time-based)
- Win condition detects board completion only in normal mode
- Track all keystrokes with timestamps in useGame
- Update example app with full scoreboard display
- Expose boardRef in BoardManager for state checking
- Adjust easy mode to 5 removed cells for testing

src/useBoard/SudokuGenerator.ts

@@ -1,7 +1,7 @@
 export type Difficulty = 'easy' | 'medium' | 'hard';
 
 const CELLS_TO_REMOVE = {
-  easy: 40,
+  easy: 5,
   medium: 50,
   hard: 60,
 } as const;