feat: 新增桌面应用支持

- 新增 desktop 应用入口，将后端与前端打包为单一可执行文件
- 集成系统托盘功能（getlantern/systray）
- 支持单实例锁和端口冲突检测
- 启动时自动打开浏览器显示管理界面
- 新增 embedfs 模块嵌入静态资源
- 新增跨平台构建脚本（macOS/Windows/Linux）
- 新增 macOS .app 打包脚本
- 统一 Makefile，移除 backend/Makefile
- 更新 README 添加桌面应用使用说明

scripts/build/package-macos.sh

@@ -0,0 +1,68 @@
+#!/bin/bash
+
+VERSION="1.0.0"
+APP_NAME="Nex"
+BUNDLE_ID="io.nex.gateway"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+PROJECT_ROOT="$(cd "${SCRIPT_DIR}/../.." && pwd)"
+BUILD_DIR="${PROJECT_ROOT}/build"
+ASSETS_DIR="${PROJECT_ROOT}/assets"
+
+echo "打包 macOS .app..."
+
+mkdir -p "${BUILD_DIR}/${APP_NAME}.app/Contents/MacOS"
+mkdir -p "${BUILD_DIR}/${APP_NAME}.app/Contents/Resources"
+
+if [ -f "${BUILD_DIR}/nex-darwin-arm64" ]; then
+    cp "${BUILD_DIR}/nex-darwin-arm64" "${BUILD_DIR}/${APP_NAME}.app/Contents/MacOS/nex"
+elif [ -f "${BUILD_DIR}/nex-darwin-amd64" ]; then
+    cp "${BUILD_DIR}/nex-darwin-amd64" "${BUILD_DIR}/${APP_NAME}.app/Contents/MacOS/nex"
+else
+    echo "错误: 未找到 macOS 二进制文件，请先运行 make desktop-darwin"
+    exit 1
+fi
+
+if [ -f "${ASSETS_DIR}/AppIcon.icns" ]; then
+    cp "${ASSETS_DIR}/AppIcon.icns" "${BUILD_DIR}/${APP_NAME}.app/Contents/Resources/"
+else
+    echo "警告: 未找到 AppIcon.icns"
+fi
+
+cat > "${BUILD_DIR}/${APP_NAME}.app/Contents/Info.plist" << EOF
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>CFBundleDevelopmentRegion</key>
+    <string>zh_CN</string>
+    <key>CFBundleExecutable</key>
+    <string>nex</string>
+    <key>CFBundleIconFile</key>
+    <string>AppIcon</string>
+    <key>CFBundleIdentifier</key>
+    <string>${BUNDLE_ID}</string>
+    <key>CFBundleInfoDictionaryVersion</key>
+    <string>6.0</string>
+    <key>CFBundleName</key>
+    <string>${APP_NAME} Gateway</string>
+    <key>CFBundleDisplayName</key>
+    <string>${APP_NAME} Gateway</string>
+    <key>CFBundlePackageType</key>
+    <string>APPL</string>
+    <key>CFBundleShortVersionString</key>
+    <string>${VERSION}</string>
+    <key>CFBundleVersion</key>
+    <string>${VERSION}</string>
+    <key>LSMinimumSystemVersion</key>
+    <string>10.13</string>
+    <key>LSUIElement</key>
+    <true/>
+    <key>NSHighResolutionCapable</key>
+    <true/>
+</dict>
+</plist>
+EOF
+
+chmod +x "${BUILD_DIR}/${APP_NAME}.app/Contents/MacOS/nex"
+
+echo "打包完成: ${BUILD_DIR}/${APP_NAME}.app"