Enhance lesson editor: prev/next nav + richer toolbar

- page.tsx: fetch sibling lessons, pass prevLesson/nextLesson props
- LessonEditor: ChevronLeft/Right nav buttons with lesson title tooltip
- Toolbar: added Underline, Strikethrough, inline Code, H1, Horizontal rule,
  Link dialog (prompt), labeled buttons for better discoverability
- Install @tiptap/extension-underline

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

src/app/admin/courses/[courseId]/modules/[moduleId]/lessons/[lessonId]/page.tsx

@@ -12,19 +12,30 @@ interface Props {
 export default async function LessonEditorPage({ params }: Props) {
   const { courseId, moduleId, lessonId } = await params;
 
-  const lesson = await prisma.lesson.findUnique({
-    where: { id: lessonId },
-    include: {
-      files: { orderBy: { createdAt: "asc" } },
-      homework: true,
-      module: {
-        include: { course: { select: { title: true, slug: true } } },
+  const [lesson, siblings] = await Promise.all([
+    prisma.lesson.findUnique({
+      where: { id: lessonId },
+      include: {
+        files: { orderBy: { createdAt: "asc" } },
+        homework: true,
+        module: {
+          include: { course: { select: { title: true, slug: true } } },
+        },
       },
-    },
-  });
+    }),
+    prisma.lesson.findMany({
+      where: { moduleId },
+      orderBy: { order: "asc" },
+      select: { id: true, title: true },
+    }),
+  ]);
 
   if (!lesson || lesson.moduleId !== moduleId) notFound();
 
+  const idx = siblings.findIndex((l) => l.id === lessonId);
+  const prevLesson = idx > 0 ? siblings[idx - 1] : null;
+  const nextLesson = idx < siblings.length - 1 ? siblings[idx + 1] : null;
+
   return (
     <div className="p-8 max-w-4xl">
       <nav className="text-xs mb-6 uppercase tracking-widest" style={{ color: "var(--muted-foreground)" }}>
@@ -50,6 +61,8 @@ export default async function LessonEditorPage({ params }: Props) {
           courseId={courseId}
           moduleId={moduleId}
           courseSlug={lesson.module.course.slug}
+          prevLesson={prevLesson}
+          nextLesson={nextLesson}
         />
       </div>