fix: set COMPOSE_CMD default in config section

Fixes "COMPOSE_CMD: unbound variable" error when running
--uninstall or --update flags. Previously the variable was only
set during check_prerequisites(), which runs after flag handling.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

install.sh

@@ -23,6 +23,7 @@ DEFAULT_MODEL="llama3.2"
 FRONTEND_PORT=7842
 BACKEND_PORT=9090
 OLLAMA_PORT=11434
+COMPOSE_CMD="docker compose"
 
 # Colors (disabled if not a terminal)
 if [[ -t 1 ]]; then