feat: Private Safe encrypted local storage, .linea export/import, and contact migration

- Store private contacts exclusively in Room DB with isPrivate flag
- Add PrivateVaultExportImportManager with AES-256-GCM encrypted .linea files
- Support PIN-based or custom password encryption for exports
- Move contacts between private safe and Google/Phone storage
- Delete system contacts when moved to private safe
- Exclude private contacts from unencrypted backups
- Add activeSessionPin tracking in PrivateVaultSecurityManager
- Add private contact banner in ContactCreateEditSheet
- Fix preferredSimSlot param error in moveContactToPublicStorage