Enhance install-homebrew.sh with user validation

Added error handling and user check for Homebrew installation.

files/usr/local/bin/install-homebrew.sh

@@ -1,7 +1,20 @@
 #!/usr/bin/env bash
-set -e
+set -euo pipefail
+
+if [[ -z "${1:-}" ]]; then
+  echo "Kein Benutzer angegeben – breche ab." >&2
+  exit 1
+fi
 
 USER="$1"
 
+# Brew bereits vorhanden?
+if sudo -u "$USER" command -v brew >/dev/null 2>&1; then
+  echo "Homebrew ist bereits installiert – überspringe."
+  exit 0
+fi
+
+echo "Installiere Homebrew für Benutzer: $USER"
+
 sudo -u "$USER" NONINTERACTIVE=1 \
   /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"