Add coverImage poster to player, fix TipTap v3 editor reset, quiz admin preview

- Add coverImage field to Lesson model (prisma)
- Pass coverImage as poster prop to KinescopePlayer
- Show quiz in read-only preview mode for admin on lesson page
- Fix TipTap v3 editor reset on save: pass [lesson.id] as deps to useEditor
  to prevent setOptions() from reinitializing content on every re-render
- Replace saveLesson Server Action call with fetch PATCH /api/admin/lessons/[id]
  to avoid Next.js 16 automatic RSC refresh after Server Actions
- Simplify revalidatePath: only revalidate module page, not lesson editor page

src/app/(student)/courses/[slug]/lessons/[lessonId]/page.tsx

@@ -113,7 +113,7 @@ export default async function LessonPage({ params }: Props) {
       {/* Video */}
       {lesson.kinescopeId && (
         <div className="mb-8">
-          <KinescopePlayer videoId={lesson.kinescopeId} />
+          <KinescopePlayer videoId={lesson.kinescopeId} poster={lesson.coverImage ?? undefined} />
         </div>
       )}
 
@@ -180,17 +180,30 @@ export default async function LessonPage({ params }: Props) {
       )}
 
       {/* Quiz */}
-      {lesson.quiz && !isAdmin && (
+      {lesson.quiz && (
         <div className="mb-8">
           <p className="text-xs font-bold uppercase tracking-widest mb-3" style={{ color: "var(--muted-foreground)" }}>
-            Тест
+            Тест{isAdmin && <span className="ml-2 opacity-50">(предпросмотр)</span>}
           </p>
-          <QuizSection
-            quiz={lesson.quiz}
-            attempt={quizAttempt ? { answers: quizAttempt.answers as Record<string, string> } : null}
-            slug={slug}
-            lessonId={lessonId}
-          />
+          {isAdmin ? (
+            <div className="space-y-4 opacity-70">
+              {lesson.quiz.questions.map((q, idx) => (
+                <div key={q.id} className="space-y-1">
+                  <p className="text-sm font-medium">{idx + 1}. {q.text}</p>
+                  <div className="px-4 py-3 text-sm" style={{ border: "2px solid var(--border)", color: "var(--muted-foreground)" }}>
+                    Поле для ответа студента
+                  </div>
+                </div>
+              ))}
+            </div>
+          ) : (
+            <QuizSection
+              quiz={lesson.quiz}
+              attempt={quizAttempt ? { answers: quizAttempt.answers as Record<string, string> } : null}
+              slug={slug}
+              lessonId={lessonId}
+            />
+          )}
         </div>
       )}