fix(submission): show blocked view when instructor toggles student view

[enkr1]

Closes #5488

Problem

When "block student viewing after submission" is enabled, students see a lock screen after finalizing. But when an instructor toggles "Student View" on the same submission, they still see the full submission — the blocked view is not shown.

Root Cause

The block check only runs on the backend (submission_view_blocked?), which checks course_user.student?. Instructors are never students, so the block is never triggered — even in student view mode.

Fix

Backend (edit.json.jbuilder):

• Expose blockStudentViewingAfterSubmitted in the assessment JSON so the frontend knows the setting is enabled

Frontend (SubmissionEditIndex):

• When grader toggles to student view (graderView: false), check if the submission would be blocked (not attempting, not published, setting enabled)
• If blocked, render <BlockedSubmission /> in place of the submission content
• Assessment header + student view toggle remain visible so the instructor can switch back

Not changed

• Backend block logic for actual students is untouched
• No new dependencies

[enkr1]

Quick ping on this bug fix — the core issue is that instructors in student view bypass the block check since they're not actual students. The fix exposes the setting to the frontend so it can render the blocked view when graderView is toggled off. No backend logic for actual students is changed. Let me know if the approach makes sense!