feat(VocabService, VocabPracticeDialog, VocabLessonView): enhance vocabulary training logic and UI feedback

- Introduced methods for improved text analysis and validation in VocabService, including `_wordCount` and `_looksLikeFragmentMismatch`, to better assess learning and reference pairs.
- Updated VocabPracticeDialog to display submitted answers and correct solutions, enhancing user feedback during practice sessions.
- Enhanced VocabLessonView to ensure only trainable vocabulary pairs are processed, improving the quality of vocabulary training.
- Added localization entries for new UI elements in both English and German, ensuring clarity in user interactions.

frontend/src/i18n/locales/de/socialnetwork.json

@@ -451,6 +451,8 @@
                 "correct": "Richtig!",
                 "wrong": "Falsch.",
                 "acceptable": "Mögliche richtige Übersetzungen:",
+                "correctSolutions": "Richtige Antwort:",
+                "yourAnswer": "Deine Antwort:",
                 "stats": "Statistik",
                 "dueToday": "Heute fällig",
                 "done": "Erledigt",

frontend/src/i18n/locales/en/socialnetwork.json

@@ -451,6 +451,8 @@
                 "correct": "Correct!",
                 "wrong": "Wrong.",
                 "acceptable": "Acceptable answers:",
+                "correctSolutions": "Correct answer:",
+                "yourAnswer": "Your answer:",
                 "stats": "Stats",
                 "dueToday": "Due today",
                 "done": "Done",