feat(vocabService, VocabPracticeDialog, VocabLessonView): enhance vocabulary handling and exposure tracking

- Updated vocabService to merge extracted vocabularies and improve handling of learning and reference pairs.
- Introduced normalization and exposure tracking in VocabPracticeDialog to ensure diverse and underexposed vocabulary practice.
- Enhanced VocabLessonView with methods to identify underexposed vocabularies and adjust selection logic for improved learning outcomes.
- Implemented new constants for minimum exposure requirements to optimize vocabulary training sessions.

backend/services/vocabService.js

@@ -614,6 +614,14 @@ export default class VocabService {
     const practicalTasks = Array.isArray(lesson?.practicalTasks) ? lesson.practicalTasks : [];
     const corePatterns = Array.isArray(lesson?.corePatterns) ? lesson.corePatterns : [];
 
+    corePatterns.forEach((entry) => {
+      const pattern = this._normalizeCorePatternEntry(entry);
+      const reference = String(pattern?.target || '').trim();
+      const learning = String(pattern?.gloss || '').trim();
+      if (!learning || !reference || learning === reference) return;
+      vocabMap.set(`${learning}-${reference}`, { learning, reference });
+    });
+
     speakingPrompts.forEach((prompt, index) => {
       const learning = String(prompt?.prompt || prompt?.title || '').trim();
       const refEntry = corePatterns[index] ?? corePatterns[0];
@@ -1488,9 +1496,12 @@ export default class VocabService {
     const extractedFromExercises = this._extractTrainerVocabsFromExercises(
       (lesson.grammarExercises || []).map((exercise) => exercise.get({ plain: true }))
     );
-    const vocabs = extractedFromExercises.length > 0
-      ? extractedFromExercises
-      : this._extractTrainerVocabsFromLessonDidactics(lesson.get({ plain: true }));
+    const fallbackVocabs = this._extractTrainerVocabsFromLessonDidactics(lesson.get({ plain: true }));
+    const mergedVocabs = new Map();
+    [...fallbackVocabs, ...extractedFromExercises].forEach((entry) => {
+      if (!entry?.learning || !entry?.reference) return;
+      mergedVocabs.set(`${entry.learning}-${entry.reference}`, entry);
+    });
 
     return {
       lesson: {
@@ -1499,7 +1510,7 @@ export default class VocabService {
         courseId: lesson.courseId,
         courseTitle: lesson.course.title
       },
-      vocabs
+      vocabs: Array.from(mergedVocabs.values())
     };
   }