feat: 初始化云笔记项目

功能特性:
- Markdown 编辑与实时预览
- 代码语法高亮
- 目录树形结构管理
- 图片粘贴上传
- Markdown 文件导入导出
- 笔记密码保护
- 前后端分离架构

技术栈:
- Go + Gin + GORM + SQLite
- 原生 HTML/CSS/JavaScript
- Highlight.js

handler/admin_handler.go

@@ -0,0 +1,88 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"note-manager/config"
+	"note-manager/service"
+)
+
+// AdminHandler 后台管理处理器
+type AdminHandler struct {
+	noteSvc *service.NoteService
+	config  *config.Config
+}
+
+// NewAdminHandler 创建后台管理处理器
+func NewAdminHandler(noteSvc *service.NoteService, cfg *config.Config) *AdminHandler {
+	return &AdminHandler{noteSvc: noteSvc, config: cfg}
+}
+
+// Login 登录页面
+func (h *AdminHandler) LoginPage(c *gin.Context) {
+	c.HTML(http.StatusOK, "login.html", gin.H{
+		"title": "后台管理登录",
+	})
+}
+
+// Login 验证登录
+func (h *AdminHandler) Login(c *gin.Context) {
+	password := c.PostForm("password")
+	if password == h.config.AdminPass {
+		// 设置 cookie，有效期 7 天
+		c.SetCookie("admin_token", "authenticated", 7*24*3600, "/", "", false, true)
+		c.JSON(http.StatusOK, gin.H{
+			"code":    0,
+			"message": "登录成功",
+		})
+		return
+	}
+	c.JSON(http.StatusUnauthorized, gin.H{
+		"code":    401,
+		"message": "密码错误",
+	})
+}
+
+// Logout 登出
+func (h *AdminHandler) Logout(c *gin.Context) {
+	c.SetCookie("admin_token", "", -1, "/", "", false, true)
+	c.JSON(http.StatusOK, gin.H{
+		"code":    0,
+		"message": "已退出登录",
+	})
+}
+
+// CheckAuth 检查是否已登录
+func (h *AdminHandler) CheckAuth(c *gin.Context) {
+	token, err := c.Cookie("admin_token")
+	if err == nil && token == "authenticated" {
+		c.JSON(http.StatusOK, gin.H{
+			"code":    0,
+			"message": "已登录",
+			"data": gin.H{
+				"authenticated": true,
+			},
+		})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{
+		"code":    0,
+		"message": "未登录",
+		"data": gin.H{
+			"authenticated": false,
+		},
+	})
+}
+
+// IndexPage 后台管理首页
+func (h *AdminHandler) IndexPage(c *gin.Context) {
+	token, err := c.Cookie("admin_token")
+	if err != nil || token != "authenticated" {
+		c.Redirect(http.StatusFound, "/admin/login")
+		return
+	}
+	c.HTML(http.StatusOK, "index.html", gin.H{
+		"title": "笔记管理后台",
+	})
+}