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/sql/add-native-language-to-courses.sql

@@ -0,0 +1,24 @@
+-- ============================================
+-- Füge native_language_id zu vocab_course hinzu
+-- ============================================
+-- Dieses Feld speichert die Muttersprache des Lerners
+-- z.B. "Bisaya für Deutschsprachige" -> language_id = Bisaya, native_language_id = Deutsch
+
+-- Spalte hinzufügen
+ALTER TABLE community.vocab_course 
+ADD COLUMN IF NOT EXISTS native_language_id INTEGER;
+
+-- Foreign Key Constraint hinzufügen
+ALTER TABLE community.vocab_course
+ADD CONSTRAINT vocab_course_native_language_fk
+FOREIGN KEY (native_language_id)
+REFERENCES community.vocab_language(id)
+ON DELETE SET NULL;
+
+-- Index für bessere Performance
+CREATE INDEX IF NOT EXISTS vocab_course_native_language_idx
+ON community.vocab_course(native_language_id);
+
+-- Kommentar hinzufügen
+COMMENT ON COLUMN community.vocab_course.native_language_id IS 
+'Muttersprache des Lerners (z.B. Deutsch, Englisch). NULL bedeutet "für alle Sprachen".';