feat(DayProduction, FalukantService, VocabLessonView): enhance vocabulary training and production tracking

- Added a new `completionCount` field to the DayProduction model to track the number of completed productions.
- Updated the FalukantService to aggregate completed productions using the new `completionCount` field, improving accuracy in production statistics.
- Introduced new vocabulary training features in VocabLessonView, including options to mark vocabulary as difficult and track remaining hard vocabulary, enhancing user engagement and learning effectiveness.
- Updated localization files for German and English to support new vocabulary training features, ensuring a consistent user experience across languages.

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

@@ -825,6 +825,13 @@
                 "trainerProgressNewContent": "Neue Inhalte: {current}/{target}",
                 "trainerProgressReview": "Wiederholung: {count}",
                 "trainerProgressMixShare": "Mischanteil: {percent}%",
+                "markVocabHard": "Als schwer markieren",
+                "markVocabHardSaved": "Vokabel als schwer markiert.",
+                "unmarkVocabHard": "Aus Schwerliste entfernen",
+                "unmarkVocabHardSaved": "Vokabel aus der Schwerliste entfernt.",
+                "hardVocabModeActive": "Intensivblock: schwere Vokabeln",
+                "hardVocabRemaining": "Offen bis gefestigt: {count}",
+                "startHardVocabTrainer": "Schwere Vokabeln trainieren ({count})",
                 "unknownExerciseTypeNotice": "Dieser Übungstyp wird in der aktuellen Ansicht noch nicht interaktiv dargestellt.",
                 "unknownExerciseTypeLabel": "Typ: {type}",
                 "lessonReviewHeadlineDone": "Diese Lektion ist in der freien Vertiefung angekommen.",

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

@@ -825,6 +825,13 @@
                 "trainerProgressNewContent": "New content: {current}/{target}",
                 "trainerProgressReview": "Review: {count}",
                 "trainerProgressMixShare": "Mixed share: {percent}%",
+                "markVocabHard": "Mark as difficult",
+                "markVocabHardSaved": "Vocabulary marked as difficult.",
+                "unmarkVocabHard": "Remove from difficult list",
+                "unmarkVocabHardSaved": "Vocabulary removed from difficult list.",
+                "hardVocabModeActive": "Intensive block: difficult vocabulary",
+                "hardVocabRemaining": "Remaining until stable: {count}",
+                "startHardVocabTrainer": "Train difficult vocabulary ({count})",
                 "unknownExerciseTypeNotice": "This exercise type is not displayed interactively in the current view yet.",
                 "unknownExerciseTypeLabel": "Type: {type}",
                 "lessonReviewHeadlineDone": "This lesson has reached the free practice stage.",