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/useBoard.ts

@@ -56,11 +56,7 @@ export function useBoard() {
 
   return {
     containerRef,
-    generatorRef,
-    rendererRef,
     boardRef,
-    solutionRef,
-    initialBoardRef,
     boardRendered,
     generateBoard,
     renderBoard,