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.

backend/migrations/20260115000002-add-course-structure.cjs

@@ -0,0 +1,47 @@
+/* eslint-disable */
+'use strict';
+
+module.exports = {
+  async up(queryInterface) {
+    // chapter_id optional machen (nicht alle Lektionen brauchen ein Kapitel)
+    await queryInterface.sequelize.query(`
+      ALTER TABLE community.vocab_course_lesson
+      ALTER COLUMN chapter_id DROP NOT NULL;
+    `);
+
+    // Kurs-Wochen/Module hinzufügen
+    await queryInterface.sequelize.query(`
+      ALTER TABLE community.vocab_course_lesson
+      ADD COLUMN IF NOT EXISTS week_number INTEGER,
+      ADD COLUMN IF NOT EXISTS day_number INTEGER,
+      ADD COLUMN IF NOT EXISTS lesson_type TEXT DEFAULT 'vocab',
+      ADD COLUMN IF NOT EXISTS audio_url TEXT,
+      ADD COLUMN IF NOT EXISTS cultural_notes TEXT;
+    `);
+
+    // Indizes für Wochen/Tage
+    await queryInterface.sequelize.query(`
+      CREATE INDEX IF NOT EXISTS vocab_course_lesson_week_idx
+        ON community.vocab_course_lesson(course_id, week_number);
+      CREATE INDEX IF NOT EXISTS vocab_course_lesson_type_idx
+        ON community.vocab_course_lesson(lesson_type);
+    `);
+
+    // Kommentar hinzufügen für lesson_type
+    await queryInterface.sequelize.query(`
+      COMMENT ON COLUMN community.vocab_course_lesson.lesson_type IS 
+        'Type: vocab, grammar, conversation, culture, review';
+    `);
+  },
+
+  async down(queryInterface) {
+    await queryInterface.sequelize.query(`
+      ALTER TABLE community.vocab_course_lesson
+      DROP COLUMN IF EXISTS week_number,
+      DROP COLUMN IF EXISTS day_number,
+      DROP COLUMN IF EXISTS lesson_type,
+      DROP COLUMN IF EXISTS audio_url,
+      DROP COLUMN IF EXISTS cultural_notes;
+    `);
+  }
+};