Prevent copying assignments

[cqnykamp]

Summary

Closes #2614

• Backend: copyContent now throws InvalidRequestError("Cannot copy an assignment") when any of the requested content IDs is an assignment root (isAssignmentRoot = true). Previously, copying an assignment was silently allowed, producing a regular activity without assignment data.
• Frontend: The "Make a copy" button in the Activities page action bar is now disabled whenever any selected item is an assignment (has assignmentInfo set).
• Tests: Two new API unit tests in copy_move.test.ts verify that copying a sequence-based assignment and a singleDoc assignment both throw the expected error.

Test plan

• Select a non-assignment activity and verify "Make a copy" is still enabled and works
• Select an assignment and verify "Make a copy" button is disabled
• API test: cannot copy an assignment passes in CI
• API test: cannot copy an assignment (singleDoc) passes in CI

https://claude.ai/code/session_019HTqQkPWkZ35RM1x1HRQ2W

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Generated by Claude Code

[cqnykamp]

What happens when a folder that contains an assignment is copied? Is this behavior checked by a test?

[cqnykamp]

Copying a folder that contains an assignment still succeeds — only the ids passed directly to copyContent are checked for isAssignmentRoot. The copy of the nested assignment comes out as a plain, unassigned activity: copySingleContent omits isAssignmentRoot, assignmentOpenOn, assignmentClosedOn, and classCode, so the new row has no assignment data (and no class code / student responses). The original assignment is untouched.

That matches the frontend, where the "Make a copy" button is only disabled when a selected item itself has assignmentInfo — a folder doesn't.

It wasn't covered by a test, so I added one in 760eb48: copying a folder containing an assignment drops the assignment data. It asserts the folder copy succeeds, the copied child has assignmentInfo === undefined, and the original is still assigned.

I couldn't run the API tests locally (no DB reachable from this sandbox); it typechecks, and CI will exercise it.