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/composables/useExerciseParse.ts

@@ -0,0 +1,31 @@
+import { Exercise } from "utils/types"
+
+type Segment =
+  | { type: "md"; content: string }
+  | { type: "exercise"; exercise: Exercise }
+
+export function parseExercises(content: string, exercises: Exercise[]): Segment[] {
+  const exerciseMap = new Map(exercises.map((e) => [e.id, e]))
+  const segments: Segment[] = []
+  const regex = /\[\[exercise:(\d+)\]\]/g
+  let lastIndex = 0
+  let match: RegExpExecArray | null
+
+  while ((match = regex.exec(content)) !== null) {
+    if (match.index > lastIndex) {
+      segments.push({ type: "md", content: content.slice(lastIndex, match.index) })
+    }
+    const id = parseInt(match[1])
+    const exercise = exerciseMap.get(id)
+    if (exercise) {
+      segments.push({ type: "exercise", exercise })
+    }
+    lastIndex = regex.lastIndex
+  }
+
+  if (lastIndex < content.length) {
+    segments.push({ type: "md", content: content.slice(lastIndex) })
+  }
+
+  return segments
+}