Add save and preview icons to lesson editor

- Save button now shows floppy disk icon (lucide Save)
- New Preview button with eye icon opens lesson in student view (new tab)
- Pass courseSlug through to LessonEditor for preview URL construction

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

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

@@ -16,7 +16,7 @@ export default async function LessonEditorPage({ params }: Props) {
     include: {
       files: { orderBy: { createdAt: "asc" } },
       module: {
-        include: { course: { select: { title: true } } },
+        include: { course: { select: { title: true, slug: true } } },
       },
     },
   });
@@ -47,6 +47,7 @@ export default async function LessonEditorPage({ params }: Props) {
           }}
           courseId={courseId}
           moduleId={moduleId}
+          courseSlug={lesson.module.course.slug}
         />
       </div>