Add portable backup restore

[marpaia]

Summary

• Back up consistent snapshots of the GG, SBOL, and Flapjack databases together with installed skills.
• Add portable restore commands that accept an exported backup directory and a separately stored recovery-key file.
• Add an explicit Settings flow to browse, inspect, verify, and stage an external snapshot.
• Preserve recipient machine-local backup, platform, and workspace identity while restoring donor application data.
• Keep current app data as a recoverable pre-restore directory and warn when older snapshots omit newer databases.

Root cause and impact

Backups previously covered only the primary database and configured local snapshots. There was no path to browse a backup supplied by another user, unlock it with its exported recovery key, or restore the complete GG-owned state.

Users can now transfer an exported backup directory and recovery key to another GG installation, inspect available snapshots, and stage a complete restore without automatically loading local snapshots.

OS-keychain secrets and external project directories remain local to the recipient device.

Validation

• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• pnpm --dir apps/desktop lint
• pnpm --dir apps/desktop build
• Rendered Settings and portable-restore dialog inspection

Fixes #8