f-1193 Add bulk edit actions to the assignment list

[morozov-av]

Adds bulk assignment management to Assignment Builder

1. Select individual assignments or all visible assignments
2. Bulk change visibility, late-submission, and delete assignments with confirmation
3. Clear selection when search filters change to prevent actions on hidden rows
4. Add unit and component coverage for the new behavior

@bnmnetp
Known concern
bulkUpdateAssignments currently sends one PUT request per assignment. Selecting many assignments may create a large request burst and partial failures.
The best fix would be to add a backend endpoint that updates multiple assignments at once.

f-1193.mp4