[SandObserver]

Summary

• Add snapshotItems and saveWithRevert to admin-save-logic.js, and a saveOrRevert wrapper in admin.js that restores the item list and re-renders when a write does not reach the server.
• Take a snapshot before every optimistic list mutation: delete, move up/down, both drag-drop paths, the Settings hide toggle, and the four folder-picker membership changes.
• Gate the "Updated"/"Added" toast and closeModal in doSave on the save result, so a failed write leaves the editor open with the form intact instead of reporting success over its own error toast.
• Gate the "Imported" toast in the JSON import path on the save result, and revert the replaced item list when the write fails.
• Keep the edit view open on a failed delete, rather than returning to a list that shows the item as gone.
• Wrap saveLabels in a catch that restores the toggled checkbox and reports the error, instead of leaving an unhandled rejection and a switch the server never agreed to.

Testing
All green. Added 6 tests covering snapshotItems cloning and saveWithRevert on success, reported failure, a throw, and a void return.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.