fix: show 404 on task detail page when the task does not exist (#1014)

frontend/cypress/e2e/task/task.spec.ts

@@ -189,7 +189,13 @@ describe('Task', () => {
 			LabelTaskFactory.truncate()
 			TaskAttachmentFactory.truncate()
 		})
+		it('Shows a 404 page for nonexisting tasks', () => {
 
+			cy.visit('/tasks/9999')
+
+			cy.contains('Not found')
+				.should('be.visible')
+		})
 		it('Shows all task details', () => {
 			const tasks = TaskFactory.create(1, {
 				id: 1,

frontend/src/views/tasks/TaskDetailView.vue

@@ -743,6 +743,13 @@ watch(
 			attachmentStore.set(task.value.attachments)
 			taskColor.value = task.value.hexColor
 			setActiveFields()
+		} catch (e) {
+			if (e?.response?.status === 404) {
+				router.replace({name: 'not-found'})
+				return
+			}
+
+			throw e
 		} finally {
 			await nextTick()
 			scrollToHeading()