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/admin/sortable-modules.tsx

@@ -17,7 +17,7 @@ import {
 } from "@dnd-kit/sortable";
 import { CSS } from "@dnd-kit/utilities";
 import Link from "next/link";
-import { createModule, deleteModule, updateModule, reorderModules } from "@/app/admin/courses/[courseId]/actions";
+import { createModule, deleteModule, updateModule, reorderModules } from "@/lib/actions/course-actions";
 
 interface Module {
   id: string;