fix(project): correctly load background when switching from or to a project view

Resolves https://community.vikunja.io/t/background-does-not-load/1437

src/views/tasks/TaskDetailView.vue

@@ -539,6 +539,12 @@ const taskId = toRef(props, 'taskId')
 
 const project = computed(() => projectStore.projects[task.value.projectId])
 watchEffect(() => {
+	if (typeof project.value === 'undefined') {
+		// assuming the task has not been loaded completely and thus the project id is 0.
+		// This avoids flickering between a project background and none when opening the task detail view from 
+		// any the project views.
+		return
+	}
 	baseStore.handleSetCurrentProject({
 		project: project.value,
 	})