fix: address code review issues in interactive exercises

- ExerciseSort: replace unbounded shuffle recursion with deterministic swap
- ExerciseManager: add sortQuestion field so sort exercises have custom questions
- index.vue: use Promise.allSettled so exercise fetch failure doesn't break lesson content
- ExerciseManager: guard mcqAnswer index after option deletion

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/oj/learn/index.vue

@@ -147,9 +147,9 @@ async function init() {
   titles.value = res1.data
   if (titles.value.length === 0) return
   const id = titles.value[step.value - 1].id
-  const [res2, exs] = await Promise.all([getTutorial(id), getExercises(id)])
-  tutorial.value = res2.data
-  exercises.value = exs
+  const [res2, exs] = await Promise.allSettled([getTutorial(id), getExercises(id)])
+  if (res2.status === "fulfilled") tutorial.value = res2.value.data
+  exercises.value = exs.status === "fulfilled" ? exs.value : []
 }
 
 watch(