feat(sync): include star and block flags in backup

[tomtom-bot7]

Summary

• Backup export is now v1.3 and includes problemFlags (neet_id, starred, blocked) for any starred or blocked problems.
• Import restores those flags when problemFlags is present (clears then reapplies). Older v1.2 backups without the field leave local flags alone.
• Auto-sync also runs after star/block toggles (same as after a review).

Test plan

• Star and block a few problems; export progress JSON and confirm problemFlags lists them.
• On another/fresh profile (or after clearing flags), import that file and confirm stars/blocks return.
• Import an older v1.2 backup without problemFlags and confirm existing local stars/blocks are unchanged.
• With auto-sync enabled, toggle a star and confirm the export file updates.