Auto-save draft every N seconds

[Mandip77]

Summary

Periodically save to a temp location to prevent data loss.

Tasks

• Use a Timeline to trigger every 30–60s when dirty.
• Save to <home>/.simple-text-editor/auto/<timestamp>.txt or alongside current file.
• On startup, detect autosave files and offer recovery.

Acceptance Criteria

• Drafts are created when editing.
• Recovery prompt appears on next launch if unsaved draft exists.