Fix all Server Actions imported from dynamic route paths

All admin and student Client Components were importing Server Actions
from paths with dynamic segments ([courseId], [moduleId], [lessonId], [slug]).
This caused "Cannot access toStringTag on the server" RSC crash.

Consolidated all Server Actions into static files under src/lib/actions/:
- course-actions.ts  (modules + enrollment)
- module-actions.ts  (lessons + reorder + move)
- user-actions.ts    (bulk grant / revoke)
- student-actions.ts (progress + homework + comments)

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

src/components/student/lesson-comments.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState, useTransition } from "react";
-import { addComment, deleteComment } from "@/app/(student)/courses/[slug]/lessons/[lessonId]/comment-actions";
+import { addComment, deleteComment } from "@/lib/actions/student-actions";
 
 type Comment = {
   id: string;