Polish setup wizard and add config check

[magrathean-uk]

Hey,

What I changed:

• kept setup focused on generating a backup profile, using TOML serialization and safe profile handling
• added rustic setup --print so the generated TOML can be inspected without writing config or creating repo dirs
• added rustic check-config to validate merged config and backup shape without touching the repo
• refreshed local, S3, SFTP, and rclone examples with placeholder-safe config

Tested:

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test setup --lib
• cargo test --test backup_restore
• cargo test --test completions
• cargo test --test check_config
• cargo test --test config

I kept this scoped away from restore prompts, init summaries, progress edits, and manual timing bits so it should be easier to review.