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/models/community/vocab_grammar_exercise.js

@@ -0,0 +1,69 @@
+import { Model, DataTypes } from 'sequelize';
+import { sequelize } from '../../utils/sequelize.js';
+
+class VocabGrammarExercise extends Model {}
+
+VocabGrammarExercise.init({
+    id: {
+        type: DataTypes.INTEGER,
+        primaryKey: true,
+        autoIncrement: true
+    },
+    lessonId: {
+        type: DataTypes.INTEGER,
+        allowNull: false,
+        field: 'lesson_id'
+    },
+    exerciseTypeId: {
+        type: DataTypes.INTEGER,
+        allowNull: false,
+        field: 'exercise_type_id'
+    },
+    exerciseNumber: {
+        type: DataTypes.INTEGER,
+        allowNull: false,
+        field: 'exercise_number'
+    },
+    title: {
+        type: DataTypes.TEXT,
+        allowNull: false
+    },
+    instruction: {
+        type: DataTypes.TEXT,
+        allowNull: true
+    },
+    questionData: {
+        type: DataTypes.JSONB,
+        allowNull: false,
+        field: 'question_data'
+    },
+    answerData: {
+        type: DataTypes.JSONB,
+        allowNull: false,
+        field: 'answer_data'
+    },
+    explanation: {
+        type: DataTypes.TEXT,
+        allowNull: true
+    },
+    createdByUserId: {
+        type: DataTypes.INTEGER,
+        allowNull: false,
+        field: 'created_by_user_id'
+    },
+    createdAt: {
+        type: DataTypes.DATE,
+        allowNull: false,
+        defaultValue: DataTypes.NOW,
+        field: 'created_at'
+    }
+}, {
+    sequelize,
+    modelName: 'VocabGrammarExercise',
+    tableName: 'vocab_grammar_exercise',
+    schema: 'community',
+    timestamps: false,
+    underscored: true
+});
+
+export default VocabGrammarExercise;