feat: add dev-daemon target for background development mode

Add a new make dev-daemon target that allows running DeerFlow services
in background mode without keeping the terminal connection.

Following the pattern of PR #1042, the implementation uses a dedicated
shell script (scripts/start-daemon.sh) for better maintainability.

- Create scripts/start-daemon.sh for daemon mode startup
- Add dev-daemon target to Makefile
- Each service writes logs to separate files (langgraph, gateway, frontend, nginx)
- Services can be stopped with make stop
- Use nohup for proper daemon process detachment
- Add cleanup on failure when services fail to start
- Use more specific pkill pattern to avoid killing unrelated nginx processes

Makefile

@@ -1,6 +1,6 @@
 # DeerFlow - Unified Development Environment
 
-.PHONY: help config check install dev stop clean docker-init docker-start docker-stop docker-logs docker-logs-frontend docker-logs-gateway
+.PHONY: help config check install dev dev-daemon stop clean docker-init docker-start docker-stop docker-logs docker-logs-frontend docker-logs-gateway
 
 help:
 	@echo "DeerFlow Development Commands:"
@@ -9,6 +9,7 @@ help:
 	@echo "  make install         - Install all dependencies (frontend + backend)"
 	@echo "  make setup-sandbox   - Pre-pull sandbox container image (recommended)"
 	@echo "  make dev             - Start all services (frontend + backend + nginx on localhost:2026)"
+	@echo "  make dev-daemon      - Start all services in background (daemon mode)"
 	@echo "  make stop            - Stop all running services"
 	@echo "  make clean           - Clean up processes and temporary files"
 	@echo ""
@@ -153,6 +154,10 @@ setup-sandbox:
 dev:
 	@./scripts/start.sh
 
+# Start all services in daemon mode (background)
+dev-daemon:
+	@./scripts/start-daemon.sh
+
 # Stop all services
 stop:
 	@echo "Stopping all services..."