fix(mongo): dirty document editor is discarded on breadcrumb Back

[ZhuchkaTriplesix]

Summary

MongoDocumentEditor tracks _dirty / _saving. Breadcrumb Back in MongoDocumentsView sets _editingDoc = null with no confirm. Same class of data loss as Table Browser Home/Close.

Scope

• If _dirty, confirm before leaving the editor (Back, tree navigation, Home).
• Align with UnsavedWorkRegistry if that is the app-wide pattern.

Out of scope

• Table Browser discard gate (separate issue).

[ZhuchkaTriplesix]

Shipped in #862