feat(03-02): implement word mode for typing engine

- Add createWordExerciseState() that splits words into letters with boundaries
- Add getCurrentWordIndex() helper for UI word tracking
- Extend TypingExerciseState with mode, words, wordBoundaries fields
- Backwards compatible: createExerciseState sets mode='letters'
- All 85 tests pass

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/types.ts

@@ -74,6 +74,9 @@ export interface TypingExerciseState {
 	correctStreak: number;
 	totalCorrect: number;
 	totalErrors: number;
+	mode: "letters" | "words";
+	words?: string[];
+	wordBoundaries?: number[];
 }
 
 // Screen names