Add cross-module lesson navigation and admin edit button

- Admin lesson editor now navigates across module boundaries (full course flat list)
- Student lesson view shows "Edit" button for admins linking to lesson editor

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

src/app/(student)/courses/[slug]/lessons/[lessonId]/page.tsx

@@ -107,6 +107,18 @@ export default async function LessonPage({ params }: Props) {
 
   return (
     <article className="max-w-3xl mx-auto px-6 py-8">
+      {/* Admin edit bar */}
+      {isAdmin && (
+        <div className="flex items-center justify-end mb-4">
+          <Link
+            href={`/admin/courses/${lesson.module.course.id}/modules/${lesson.moduleId}/lessons/${lessonId}`}
+            className="btn-aubade text-xs px-3 py-1.5"
+          >
+            Редактировать урок
+          </Link>
+        </div>
+      )}
+
       {/* Title */}
       <h1 className="text-2xl font-bold mb-6 leading-snug">{lesson.title}</h1>