fix(mongo): confirm before discarding a dirty document editor

[ZhuchkaTriplesix]

Summary

• Dirty MongoDocumentEditor JSON registers with UnsavedWorkRegistry, so Home / Close / tree navigation confirm before discarding.
• Editor Back and explorer breadcrumbs that leave the document use the same unsaved-work dialog; Cancel keeps the editor.

Closes #782

Test plan

• Open a Mongo document, edit JSON, tap Back → Cancel keeps the editor; Discard returns to the list
• Clean (unedited) Back leaves immediately
• Dirty editor + Home / Close / another database in the tree shows the same confirm
• After Discard, Home/Close no longer warns