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
@@ -298,23 +301,29 @@ const postQuestionsSequentially = async (id, list) => {
 }
 
 const onSubmit = async () => {
-  const { isValid } = await validate(form.value)
-  const newQuestions = validateLocalQuestions()
-  if (!isValid || !newQuestions) return
+  if (isSubmitting.value) return
+  isSubmitting.value = true
+  try {
+    const { isValid } = await validate(form.value)
+    const newQuestions = validateLocalQuestions()
+    if (!isValid || !newQuestions) return
 
-  const examPayload = buildExamPayload(form.value)
-  let targetExamId = examId.value
-  if (isEditMode.value) {
-    await updateExamMutation.mutateAsync({ id: targetExamId, payload: examPayload })
-  } else {
-    const created = await addExamMutation.mutateAsync(examPayload)
-    targetExamId = created?.data?.id ?? created?.id ?? targetExamId
+    const examPayload = buildExamPayload(form.value)
+    let targetExamId = examId.value
+    if (isEditMode.value) {
+      await updateExamMutation.mutateAsync({ id: targetExamId, payload: examPayload })
+    } else {
+      const created = await addExamMutation.mutateAsync(examPayload)
+      targetExamId = created?.data?.id ?? created?.id ?? targetExamId
+    }
+    if (targetExamId && newQuestions.length > 0) {
+      await postQuestionsSequentially(targetExamId, newQuestions)
+    }
+    await queryClient.invalidateQueries({ queryKey: adminExamsKeys.all, refetchType: 'all' })
+    router.push({ name: 'admin-exams' })
+  } finally {
+    isSubmitting.value = false
   }
-  if (targetExamId && newQuestions.length > 0) {
-    await postQuestionsSequentially(targetExamId, newQuestions)
-  }
-  await queryClient.invalidateQueries({ queryKey: adminExamsKeys.all, refetchType: 'all' })
-  router.push({ name: 'admin-exams' })
 }
 
 const onCancel = () => router.push({ name: 'admin-exams' })