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

lib/common.sh

@@ -171,6 +171,42 @@ require_debian_ubuntu() {
     fi
 }
 
+# Verify enough free disk space before doing apt installs or writing config.
+# Fails early with a clear message so we don't get half-installed packages or
+# files truncated mid-write when the disk is full.
+#
+# Usage:
+#   require_disk_space                       # 500MB on /var (apt cache + lists)
+#   require_disk_space 1024                  # 1024MB on /var
+#   require_disk_space 1024 /var /opt        # 1024MB on each listed path
+#
+# Duplicate filesystems (e.g. /var on the same fs as /) are checked once.
+require_disk_space() {
+    local min_mb=${1:-500}
+    shift 2>/dev/null || true
+    local paths=("$@")
+    [ ${#paths[@]} -eq 0 ] && paths=(/var)
+
+    local path avail fs seen=" " rc=0
+    for path in "${paths[@]}"; do
+        fs=$(df -P "$path" 2>/dev/null | awk 'NR==2 {print $1}')
+        if [ -z "$fs" ]; then
+            err "Cannot read disk usage for ${path}"
+            rc=1
+            continue
+        fi
+        [[ "$seen" == *" $fs "* ]] && continue
+        seen="$seen$fs "
+
+        avail=$(df -Pm "$path" 2>/dev/null | awk 'NR==2 {print $4}')
+        if [ "${avail:-0}" -lt "$min_mb" ]; then
+            err "Not enough disk space on ${path} (${fs}): ${avail}MB free, ${min_mb}MB required."
+            rc=1
+        fi
+    done
+    return $rc
+}
+
 # Re-exec the current script via sudo if not already root. Preserves env and
 # arguments. Call near the top of a sub-script:
 #   require_root "$0" "$@"