Implement vocab course and grammar exercise features in backend and frontend

- Added new course management functionalities in VocabController, including creating, updating, and deleting courses and lessons.
- Implemented enrollment and progress tracking for courses, along with grammar exercise creation and management.
- Updated database schema to include tables for courses, lessons, enrollments, and grammar exercises.
- Enhanced frontend with new routes and views for course listing and details, including internationalization support for course-related texts.
- Improved user experience by adding navigation to courses from the main vocab trainer view.

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

@@ -319,6 +319,35 @@
                 "search": "Suchen",
                 "noResults": "Keine Treffer.",
                 "error": "Suche fehlgeschlagen."
+            },
+            "courses": {
+                "title": "Sprachlernkurse",
+                "create": "Kurs erstellen",
+                "myCourses": "Meine Kurse",
+                "allCourses": "Alle Kurse",
+                "none": "Keine Kurse gefunden.",
+                "owner": "Besitzer",
+                "enrolled": "Eingeschrieben",
+                "public": "Öffentlich",
+                "difficulty": "Schwierigkeit",
+                "lessons": "Lektionen",
+                "enroll": "Einschreiben",
+                "continue": "Fortsetzen",
+                "edit": "Bearbeiten",
+                "addLesson": "Lektion hinzufügen",
+                "completed": "Abgeschlossen",
+                "score": "Punktzahl",
+                "review": "Wiederholen",
+                "start": "Starten",
+                "noLessons": "Dieser Kurs hat noch keine Lektionen.",
+                "lessonNumber": "Lektionsnummer",
+                "chapter": "Kapitel",
+                "selectChapter": "Kapitel auswählen",
+                "selectLanguage": "Sprache auswählen",
+                "confirmDelete": "Lektion wirklich löschen?",
+                "titleLabel": "Titel",
+                "descriptionLabel": "Beschreibung",
+                "languageLabel": "Sprache"
             }
         }
     }

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

@@ -319,6 +319,35 @@
                 "search": "Search",
                 "noResults": "No results.",
                 "error": "Search failed."
+            },
+            "courses": {
+                "title": "Language Learning Courses",
+                "create": "Create Course",
+                "myCourses": "My Courses",
+                "allCourses": "All Courses",
+                "none": "No courses found.",
+                "owner": "Owner",
+                "enrolled": "Enrolled",
+                "public": "Public",
+                "difficulty": "Difficulty",
+                "lessons": "Lessons",
+                "enroll": "Enroll",
+                "continue": "Continue",
+                "edit": "Edit",
+                "addLesson": "Add Lesson",
+                "completed": "Completed",
+                "score": "Score",
+                "review": "Review",
+                "start": "Start",
+                "noLessons": "This course has no lessons yet.",
+                "lessonNumber": "Lesson Number",
+                "chapter": "Chapter",
+                "selectChapter": "Select Chapter",
+                "selectLanguage": "Select Language",
+                "confirmDelete": "Really delete lesson?",
+                "titleLabel": "Title",
+                "descriptionLabel": "Description",
+                "languageLabel": "Language"
             }
         }
     }