Add four years of German listening, reading and writing beside the textbook.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/progress.c

@@ -59,6 +59,7 @@ void load_progress(void) {
     on3_load_progress();
     on4_load_progress();
     en_load_progress();
+    de_load_progress();
     lit_load_progress();
     sci_load_progress();
     fyz_load_progress();