feat(vocab): enhance vocabService and VocabLessonView for improved exercise handling

- Updated VocabService to include validation for synthetic exercise IDs and added methods for generating multiple-choice exercises based on chapter lexemes.
- Implemented a seeded shuffle function to randomize distractor options in multiple-choice questions, ensuring varied user experiences.
- Modified VocabLessonView to adjust target calculations for lesson goals and unlock attempts, increasing the maximum limits for better flexibility in user progress tracking.

frontend/src/views/social/VocabLessonView.vue

@@ -979,7 +979,7 @@ export default {
       const durationBonus = Math.max(0, Math.round((this.lesson?.targetMinutes || 0) / 5) - 1);
       const baseTarget = Math.ceil((Math.max(vocabCount, 4) * 1.35) + (exerciseCount * 0.35) + durationBonus);
       const weightedTarget = Math.ceil(baseTarget * this.lessonComplexityWeight);
-      return Math.min(24, Math.max(6, weightedTarget));
+      return Math.max(6, Math.min(120, weightedTarget));
     },
     trainerReviewBlendStart() {
       return Math.max(3, Math.ceil(this.trainerNewFocusTarget * 0.4));
@@ -989,7 +989,7 @@ export default {
     },
     trainerExerciseUnlockAttempts() {
       const unlockTarget = this.trainerNewFocusTarget + Math.ceil((this.effectiveExercises?.length || 0) * 0.25);
-      return Math.min(28, Math.max(6, unlockTarget));
+      return Math.max(6, Math.min(140, unlockTarget));
     },
     currentReviewShare() {
       if (!this.hasPreviousVocab) {