Add settings backup, restore, and factory reset

[PurpleSentinel]

Parent: #23

Summary

Give users controlled recovery tools beyond manually replacing JSON files.

Acceptance criteria

• Create a validated backup before reset or bulk import.
• Provide explicit restore and factory-reset actions with confirmation.
• Preserve recoverability if power fails during the operation.
• Show which settings will be retained or removed.
• Add atomicity, migration, and invalid-backup tests.

Roadmap group

Data, configuration, and maintenance