fix(exams): prevent double-submit from creating duplicate exams

The submit button only disabled after Vue re-rendered on mutation
pending state, leaving a window where a fast double-click fired
onSubmit twice — creating two exam rows, with the losing request
usually failing to attach its questions (an "empty" duplicate exam).

src/features/admin/exams/pages/ExamFormPage.vue

@@ -283,8 +283,11 @@ const addExamMutation = useAddAdminExamMutation()
 const updateExamMutation = useUpdateAdminExamMutation()
 const addQuestionMutation = useAddAdminExamQuestionMutation()
 
+const isSubmitting = ref(false)
+
 const submitting = computed(
   () =>
+    isSubmitting.value ||
     addExamMutation.isPending.value ||
     updateExamMutation.isPending.value ||
     addQuestionMutation.isPending.value
@@ -298,6 +301,9 @@ const postQuestionsSequentially = async (id, list) => {
 }
 
 const onSubmit = async () => {
+  if (isSubmitting.value) return
+  isSubmitting.value = true
+  try {
     const { isValid } = await validate(form.value)
     const newQuestions = validateLocalQuestions()
     if (!isValid || !newQuestions) return
@@ -315,6 +321,9 @@ const onSubmit = async () => {
     }
     await queryClient.invalidateQueries({ queryKey: adminExamsKeys.all, refetchType: 'all' })
     router.push({ name: 'admin-exams' })
+  } finally {
+    isSubmitting.value = false
+  }
 }
 
 const onCancel = () => router.push({ name: 'admin-exams' })