Initial commit: DHCP Web Manager

Flask-based web UI for managing ISC DHCP server (dhcpd).

Features:
- subnet / static host binding CRUD via web
- listen interface configuration (INTERFACESv4)
- active lease query + pool utilization stats
- service status, reload/restart, syntax check
- user auth + operation logs
- automatic backup before each config change
- safe change pipeline: write file → dhcpd -t → systemctl restart + is-active verify
- error messages include journalctl tail for diagnosis

templates/error.html

@@ -0,0 +1,9 @@
+{% extends "base.html" %}
+{% block title %}{{ code }} 错误{% endblock %}
+{% block content %}
+<div class="card error-page">
+    <h1>{{ code }}</h1>
+    <p>{{ message }}</p>
+    <a href="{{ url_for('dashboard') }}" class="btn">返回首页</a>
+</div>
+{% endblock %}