feat(lib): add require_disk_space helper and enforce pre-flight disk checks

- add `require_disk_space` function to lib/common.sh with dedup logic for shared filesystems
- gate cloudpanel, coolify, pterodactyl installs behind a 2–3 GB disk check
- gate uptime-kuma, proxmox update, harden, update-server, and server-benchmark behind 300–1024 MB disk checks
- fail early with a clear error before apt installs or config writes can leave the system in a partial state

tools/harden.sh

@@ -56,6 +56,7 @@ done
 # ═══════════════════════════════════════════════════════════════════════════
 
 require_debian_ubuntu || exit 1
+require_disk_space 500 || exit 1
 
 # Read the effective Port from sshd_config + any drop-in under sshd_config.d/
 sshd_effective_port() {