[Feature]: Classroom Sharing Functionality

### Problem or Motivation

Currently, all generated courses are stored locally in the user's browser. When a teacher creates a course and wants to share it with students (e.g., by copying a link like xxx.xx.xxx.xx/classroom/xxxx), other users who open the link see a blank page and cannot access the course content.

Expected Behavior

Add a classroom sharing feature that allows users to share generated classrooms with others via a link. Recipients should be able to access and study the course content normally.

Current Impact

Teachers cannot distribute created courses to students

Course content is only visible to the creator, limiting teaching and sharing capabilities

Restricts the system's collaboration and dissemination potential

### Proposed Solution

Support generating shareable links for classrooms

Store course data on the server side rather than only in browser local storage

Provide sharing permission controls (public / specific users / password-protected, etc.)

### Alternatives Considered

_No response_

### Area

Classroom generation

### Additional Context

_No response_