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

apps/cloudpanel.sh

@@ -219,7 +219,8 @@ install_cloudpanel() {
         echo -e "${YELLOW}CloudPanel is already installed!${NC}"
         return 0
     fi
-    
+
+    require_disk_space 2048 || return 1
     check_requirements || return 1
     echo ""