feat: add interactive MCQ and code-sort exercise widgets to tutorial lessons

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

src/oj/learn/components/ExerciseWidget.vue

@@ -0,0 +1,13 @@
+<script setup lang="ts">
+import { Exercise } from "utils/types"
+
+const ExerciseMcq = defineAsyncComponent(() => import("./ExerciseMcq.vue"))
+const ExerciseSort = defineAsyncComponent(() => import("./ExerciseSort.vue"))
+
+defineProps<{ exercise: Exercise }>()
+</script>
+
+<template>
+  <ExerciseMcq v-if="exercise.type === 'mcq'" :exercise="exercise" />
+  <ExerciseSort v-else-if="exercise.type === 'sort'" :exercise="exercise" />
+</template>