Cloud backup status UI in settings

[praveenperera]

Expose redb cloud sync status to the UI via the existing Updater pattern.

pub struct CloudBackupStatus {
    pub total_files: u32,
    pub synced_files: u32,
    pub pending_files: u32,
    pub failed_files: u32,
    pub last_sync_at: Option<String>, // ISO 8601
}

Send Updater::send_update(Update::CloudBackupStatusChanged(status)) when upload state changes.

Settings screen shows:

• "Fully synced" / "2 files pending upload" / "Last synced: 5 min ago"
• Error state if uploads are failing persistently

Files:

• rust/src/cloud_backup_actor.rs — emit status updates
• iOS/Android settings screens — display status

Depends on: CloudBackupActor