Fix saveLesson: sanitize content JSON to prevent RSC proxy error

React treats TipTap's editor.getJSON() output as a non-plain object when
passed through Server Action serialization, causing isDecimal() inside
Prisma's query builder to receive a temporary client reference proxy and
throw ERROR 1213974697. JSON.parse(JSON.stringify()) strips the prototype
chain and any non-enumerable properties, ensuring Prisma receives a
clean plain object.

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

src/lib/actions/lesson-actions.ts

@@ -22,20 +22,15 @@ export async function saveLesson(
   }
 ) {
   await requireAdmin();
-  try {
-    await prisma.lesson.update({
-      where: { id: lessonId },
-      data: {
-        title: data.title,
-        kinescopeId: data.kinescopeId || null,
-        content: JSON.parse(JSON.stringify(data.content)),
-        published: data.published,
-      },
-    });
-  } catch (err) {
-    console.error("[saveLesson] full error:", err);
-    throw err;
-  }
+  await prisma.lesson.update({
+    where: { id: lessonId },
+    data: {
+      title: data.title,
+      kinescopeId: data.kinescopeId || null,
+      content: JSON.parse(JSON.stringify(data.content)),
+      published: data.published,
+    },
+  });
   revalidatePath(`/admin/courses/${courseId}/modules/${moduleId}`);
   revalidatePath(`/admin/courses/${courseId}/modules/${moduleId}/lessons/${lessonId}`);
 }