feat: add faculty dashboard page and delete action for assignment sheets

[ishad05]

Summary

Improves the faculty assignment dashboard with assigned-submission tracking, draft recovery, publish/reschedule actions, and assignment deletion.

Changes

• Replaced the faculty dashboard “Latest submissions” widget with an “Assigned to you” queue.
• Added assigned-submission support using Frappe’s built-in assignment/ToDo flow.
• Added ability to assign, bulk assign, and unassign submissions to faculty members.
• Added assignee display/actions on the faculty assignment detail submissions table.
• Added faculty member picker for assignment flows.
• Added publish/reschedule action for existing draft or scheduled assignments.
• Added draft recovery:
  • continue an existing draft
  • autosave draft on sheet close
  • drafts section on the assignments dashboard
  • draft prefill through a faculty-gated assignment getter
• Made assignment drafts more forgiving by allowing cohort and due date to be empty until publish/schedule.
• Added assignment delete flow with a confirmation dialog.
• Prevented deleting assignments that already have submissions.
• Added backend APIs for:
  • assigned submissions
  • assign/bulk assign/unassign submission
  • faculty member list
  • get/update/publish/delete assignment
• Added/expanded E2E coverage for assigned submissions, draft continuation/autosave, publish/reschedule, and delete behaviour.

Updates

• Made scheduled assignments show up for students exactly at their publish time instead of a few minutes late — visibility is now based on server time, and the page reveals the assignment on time without a manual refresh.
• Student assignments and grades now auto-refresh so newly published stuff shows up without reloading.
• Show the scheduled time in the status column when an assignment is scheduled.
• Dashboard "Upcoming assignments" shows the most recently published ones first.
• Added a student assignments API (published + due-now, with the next publish time).
• Fixed the assign/unassign API so it works on real Frappe (was passing an arg that only worked on our local patched copy).
• Fixed scheduled/draft grades showing up on the student dashboard the moment they were saved.