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