Add lesson progress tracking

- Toggle lesson completion via server action (LessonProgress table)
- "Отметить как пройденный" button on lesson page, turns accent when done
- Course sidebar: progress bar, checkmarks on completed lessons, X/Y counter per module
- Dashboard: progress bar on each course card with completion percentage

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

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

@@ -47,9 +47,20 @@ export default async function CourseLayout({ children, params }: Props) {
     }
   }
 
+  // Fetch completed lesson IDs for this user
+  const allLessonIds = course.modules.flatMap((m) => m.lessons.map((l) => l.id));
+  const progressRecords = isAdmin
+    ? []
+    : await prisma.lessonProgress.findMany({
+        where: { userId: session.user.id, lessonId: { in: allLessonIds } },
+        select: { lessonId: true },
+      });
+
+  const completedLessonIds = new Set(progressRecords.map((p) => p.lessonId));
+
   return (
     <div className="flex flex-1">
-      <CourseSidebar course={course} />
+      <CourseSidebar course={course} completedLessonIds={completedLessonIds} />
       <main className="flex-1 min-w-0 overflow-y-auto">
         {children}
       </main>