Add Vocab Trainer feature with routing, database schema, and translations

- Introduced Vocab Trainer functionality, including new routes for managing languages and chapters.
- Implemented database schema for vocab-related tables to ensure data integrity.
- Updated navigation and UI components to include Vocab Trainer in the social network menu.
- Added translations for Vocab Trainer in both German and English locales, enhancing user accessibility.

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

@@ -249,5 +249,66 @@
             "denied": "You have denied the friendship request.",
             "accepted": "The friendship has been established."
         }
+        ,
+        "vocab": {
+            "title": "Vocabulary trainer",
+            "description": "Create languages (or subscribe to them) and share them with friends.",
+            "newLanguage": "New language",
+            "newLanguageTitle": "Create new language",
+            "languageName": "Language name",
+            "create": "Create",
+            "saving": "Saving...",
+            "created": "Language created.",
+            "createdTitle": "Vocabulary trainer",
+            "createdMessage": "Language created. The menu will refresh.",
+            "createError": "Could not create language.",
+            "openLanguage": "Open",
+            "none": "You have no languages yet (created or subscribed).",
+            "owner": "Owned",
+            "subscribed": "Subscribed",
+            "languageTitle": "Vocabulary trainer: {name}",
+            "notFound": "Language not found or no access.",
+            "shareCode": "Share code",
+            "shareHint": "Send this code to friends so they can subscribe to this language.",
+            "subscribeByCode": "Subscribe by code",
+            "subscribeTitle": "Subscribe to language",
+            "subscribeHint": "Enter a share code you received from a friend.",
+            "subscribe": "Subscribe",
+            "subscribeSuccess": "Subscribed. The menu will refresh.",
+            "subscribeError": "Subscribe failed. Invalid code or no access.",
+            "trainerPlaceholder": "Trainer features (words/quizzes) will be the next step."
+            ,
+            "chapters": "Chapters",
+            "newChapter": "New chapter",
+            "createChapter": "Create chapter",
+            "createChapterError": "Could not create chapter.",
+            "noChapters": "No chapters yet.",
+            "chapterTitle": "Chapter: {title}",
+            "addVocab": "Add vocabulary",
+            "learningWord": "To learn",
+            "referenceWord": "Reference",
+            "add": "Add",
+            "addVocabError": "Could not add vocabulary.",
+            "noVocabs": "No vocabulary in this chapter yet."
+            ,
+            "practice": {
+                "open": "Practice",
+                "title": "Practice vocabulary",
+                "allVocabs": "All vocabulary",
+                "simple": "Simple practice",
+                "noPool": "No vocabulary to practice.",
+                "dirLearningToRef": "To learn → Reference",
+                "dirRefToLearning": "Reference → To learn",
+                "check": "Check",
+                "next": "Next",
+                "skip": "Skip",
+                "correct": "Correct!",
+                "wrong": "Wrong.",
+                "acceptable": "Acceptable answers:",
+                "stats": "Stats",
+                "success": "Success",
+                "fail": "Fail"
+            }
+        }
     }
 }