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/components/ExerciseSort.vue

@@ -22,7 +22,8 @@ function shuffle(arr: LineItem[]): LineItem[] {
     ;[a[i], a[j]] = [a[j], a[i]]
   }
   const isCorrect = a.every((item, i) => item.originalIdx === i)
-  return isCorrect && a.length > 1 ? shuffle(arr) : a
+  if (isCorrect && a.length > 1) [a[0], a[1]] = [a[1], a[0]]
+  return a
 }
 
 function init() {