Multi-tab editing warning

# Multi-tab editing warning

Detect when a student has the same document open in two tabs and warn them. Deferred from earlier work where presence detection was scoped down.

## Problem

A student opens the same document in two tabs (intentionally or by accidentally clicking from a dashboard). Edits in one tab can stomp on edits in the other depending on save timing. Today there's no warning.

## Goals (rough)

- A student opening a document in a second tab while another tab has it open sees a warning.
- The warning makes clear which tab is "active" — closing the other or making this tab active before editing.

## Open questions

- [ ] Detection: BroadcastChannel, localStorage signaling, or server-side presence?
- [ ] What happens if the student dismisses and edits anyway — last-write-wins, or block editing?
- [ ] Teacher case: is there a teacher equivalent worth covering?

## Engineering handoff

Not ready. Needs detection mechanism decision and conflict-resolution policy.