feat(admin): 新增系统设置页面和配置管理功能

- 新增 settings 表用于存储 public/private 两行 JSON 配置
- 添加 AdminSettings 相关类型定义和 API 接口
- 实现管理后台系统设置页面，支持可视化和 JSON 编辑模式
- 集成 CodeMirror 编辑器用于 JSON 配置编辑
- 添加系统设置相关的路由和接口
- 更新依赖包并修改相关样式和布局文件

handler/settings.go

@@ -0,0 +1,38 @@
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/basketikun/infinite-canvas/model"
+	"github.com/basketikun/infinite-canvas/service"
+)
+
+func Settings(w http.ResponseWriter, r *http.Request) {
+	settings, err := service.PublicSettings()
+	if err != nil {
+		Fail(w, err.Error())
+		return
+	}
+	OK(w, settings)
+}
+
+func AdminSettings(w http.ResponseWriter, r *http.Request) {
+	settings, err := service.AdminSettings()
+	if err != nil {
+		Fail(w, err.Error())
+		return
+	}
+	OK(w, settings)
+}
+
+func AdminSaveSettings(w http.ResponseWriter, r *http.Request) {
+	var settings model.Settings
+	_ = json.NewDecoder(r.Body).Decode(&settings)
+	result, err := service.SaveSettings(settings)
+	if err != nil {
+		Fail(w, err.Error())
+		return
+	}
+	OK(w, result)
+}