Make automatic page backups recoverable in the app #59

Problem

Lucid already creates bounded automatic backups before saves, but they are internal-only. A person cannot inspect or restore them after an accidental edit, crash, or damaged current page.

Scope

• expose valid backup snapshots for a page through the existing main-process library boundary
• show backup time and allow restoring one as a new page copy
• never overwrite the current page during recovery
• ignore/report unreadable backup snapshots safely
• add library, IPC, UI, and focused end-to-end coverage

Not in scope

• cloud sync or accounts
• changing the current autosave cadence or retention policy
• automatic destructive rollback