Add native language support in vocab course management

- Introduced a new field for native language in the VocabCourse model to allow learners to specify their native language.
- Updated the VocabService to handle native language during course creation and retrieval, including filtering options.
- Enhanced the database schema to include foreign key constraints for native language.
- Updated frontend components to support native language selection and display in course listings.
- Added internationalization strings for native language features in both German and English.

backend/utils/syncDatabase.js

@@ -152,6 +152,7 @@ const syncDatabase = async () => {
                     title TEXT NOT NULL,
                     description TEXT,
                     language_id INTEGER NOT NULL,
+                    native_language_id INTEGER,
                     difficulty_level INTEGER DEFAULT 1,
                     is_public BOOLEAN DEFAULT false,
                     share_code TEXT,
@@ -165,6 +166,10 @@ const syncDatabase = async () => {
                         FOREIGN KEY (language_id)
                         REFERENCES community.vocab_language(id)
                         ON DELETE CASCADE,
+                    CONSTRAINT vocab_course_native_language_fk
+                        FOREIGN KEY (native_language_id)
+                        REFERENCES community.vocab_language(id)
+                        ON DELETE SET NULL,
                     CONSTRAINT vocab_course_share_code_uniq UNIQUE (share_code)
                 );
 
@@ -239,6 +244,8 @@ const syncDatabase = async () => {
                     ON community.vocab_course(owner_user_id);
                 CREATE INDEX IF NOT EXISTS vocab_course_language_idx
                     ON community.vocab_course(language_id);
+                CREATE INDEX IF NOT EXISTS vocab_course_native_language_idx
+                    ON community.vocab_course(native_language_id);
                 CREATE INDEX IF NOT EXISTS vocab_course_public_idx
                     ON community.vocab_course(is_public);
                 CREATE INDEX IF NOT EXISTS vocab_course_lesson_course_idx