fix: reuse existing Caddy binary and preserve Caddyfile on install

src/core.sh

@@ -1293,13 +1293,18 @@ get() {
         is_host_dns=$(_wget -qO- --header="accept: application/dns-json" "https://one.one.one.one/dns-query?name=$host&type=$is_dns_type")
         ;;
     install-caddy)
-        _green "\n安装 Caddy 实现自动配置 TLS.\n"
-        load download.sh
-        download caddy
+        if [[ -f $is_caddy_bin || $(type -P caddy) ]]; then
+            [[ ! -f $is_caddy_bin ]] && is_caddy_bin=$(type -P caddy)
+            _green "\n检测到系统已安装 Caddy (${is_caddy_bin})，将直接使用现有版本。\n"
+        else
+            _green "\n安装 Caddy 实现自动配置 TLS.\n"
+            load download.sh
+            download caddy
+        fi
         load systemd.sh
         install_service caddy &>/dev/null
         is_caddy=1
-        _green "安装 Caddy 成功.\n"
+        _green "Caddy 配置成功.\n"
         ;;
     reinstall)
         is_install_sh=$(cat $is_sh_dir/install.sh)