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.

backend/models/falukant/log/dayproduction.js

@@ -23,7 +23,11 @@ DayProduction.init({
     productionDate: {
         type: DataTypes.DATEONLY,
         allowNull: false,
-        defaultValue: sequelize.literal('CURRENT_DATE')}
+        defaultValue: sequelize.literal('CURRENT_DATE')},
+    completionCount: {
+        type: DataTypes.INTEGER,
+        allowNull: false,
+        defaultValue: 1}
 }, {
     sequelize,
     modelName: 'DayProduction',