fix: Properly saving tasks

Tasks were not properly saved by the JsonTaskList, resulting in the newly created tasks being discarded immediately. Fixed by putting them into the taskMap in saveTask()

src/main/java/ch/zhaw/gartenverwaltung/io/JsonTaskList.java

@@ -106,6 +106,7 @@ public class JsonTaskList implements TaskList {
         if(task.getId() == 0) {
             task.withId(idProvider.incrementAndGet());
         }
+        taskMap.put(task.getId(), task);
         writeTaskListToFile();
     }