feat: collapsible mobile sidebar for student course view

Hamburger button (top-left, lg:hidden), dark overlay, slide-in animation.
Sidebar closes on lesson link click. Spacer added to prevent content overlap.

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

src/app/(student)/courses/[slug]/layout.tsx

@@ -62,6 +62,7 @@ export default async function CourseLayout({ children, params }: Props) {
     <div className="flex flex-1">
       <CourseSidebar course={course} completedLessonIds={completedLessonIds} />
       <main className="flex-1 min-w-0 overflow-y-auto">
+        <div className="h-12 lg:hidden" />
         {children}
       </main>
     </div>