feat: cloud sync for backups via rclone

[Christian-Gennari]

Originally reported by @Christian-Gennari in #107

Problem

Backups are created locally in ./backups/ but remain on the same machine. If the machine is lost, the backups are lost too.

Proposed Solution

Add cloud sync support using rclone as the transport layer, supporting 40+ cloud providers (S3, GCS, Azure, Dropbox, Google Drive, etc.).

New files

File	Purpose
Services/Sync/RcloneService.cs	Check rclone availability, run rclone sync, report results
Commands/SyncCommand.cs	Standalone sync command — uploads the backups folder

Changes to existing files

File	Change
Program.cs	Add --sync flag to backup command, add sync subcommand
Commands/BackupCommand.cs	Accept --sync flag; invoke sync after archive creation if enabled
Commands/InitCommand.cs	Scaffold sync.txt alongside existing config files

sync.txt config

# Cloud sync configuration
# Run `rclone config` to set up a remote first
provider=
path=

CLI usage

# One-time setup
./UbuntuSafeSnap init          # creates targets.txt, exclusions.txt, sync.txt
rclone config                  # configure a remote

# Backup + auto-sync
./UbuntuSafeSnap backup --sync

# Or sync manually anytime
./UbuntuSafeSnap sync

[Christian-Gennari]

Indefinitely postponed. The project now includes a manual cloud backup guide (docs/cloud-backup-guide.md) that covers the recommended workflow via rclone. An automated sync command may be reconsidered if demand for in-CLI cloud integration arises, but the guide is sufficient for the foreseeable future.