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]/lessons/[lessonId]/actions.ts

@@ -0,0 +1,29 @@
+"use server";
+
+import { prisma } from "@/lib/prisma";
+import { auth } from "@/lib/auth";
+import { headers } from "next/headers";
+import { revalidatePath } from "next/cache";
+
+export async function toggleLessonProgress(lessonId: string, slug: string) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session) throw new Error("Unauthorized");
+
+  const existing = await prisma.lessonProgress.findUnique({
+    where: { userId_lessonId: { userId: session.user.id, lessonId } },
+  });
+
+  if (existing) {
+    await prisma.lessonProgress.delete({
+      where: { userId_lessonId: { userId: session.user.id, lessonId } },
+    });
+  } else {
+    await prisma.lessonProgress.create({
+      data: { userId: session.user.id, lessonId },
+    });
+  }
+
+  revalidatePath(`/courses/${slug}/lessons/${lessonId}`);
+  revalidatePath(`/courses/${slug}`);
+  revalidatePath("/dashboard");
+}