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/components/student/lesson-complete-button.tsx

@@ -0,0 +1,29 @@
+"use client";
+
+import { useTransition } from "react";
+import { Check } from "lucide-react";
+import { toggleLessonProgress } from "@/app/(student)/courses/[slug]/lessons/[lessonId]/actions";
+
+export function LessonCompleteButton({
+  lessonId,
+  slug,
+  isCompleted,
+}: {
+  lessonId: string;
+  slug: string;
+  isCompleted: boolean;
+}) {
+  const [pending, startTransition] = useTransition();
+
+  return (
+    <button
+      onClick={() => startTransition(() => toggleLessonProgress(lessonId, slug))}
+      disabled={pending}
+      className={`btn-aubade ${isCompleted ? "btn-aubade-accent" : ""} flex items-center gap-2 px-5 py-2.5 text-sm`}
+      style={{ opacity: pending ? 0.6 : 1 }}
+    >
+      <Check size={15} strokeWidth={isCompleted ? 3 : 2} />
+      {pending ? "..." : isCompleted ? "Пройдено" : "Отметить как пройденный"}
+    </button>
+  );
+}