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

repository/db.go

@@ -48,6 +48,7 @@ func DB() (*gorm.DB, error) {
 			&model.User{},
 			&model.Prompt{},
 			&model.Asset{},
+			&model.Setting{},
 		)
 	})
 	return db, dbErr

repository/setting.go

@@ -0,0 +1,48 @@
+package repository
+
+import (
+	"encoding/json"
+
+	"github.com/basketikun/infinite-canvas/model"
+	"gorm.io/gorm/clause"
+)
+
+// GetSettings 返回 public 和 private 两行配置。
+func GetSettings() (model.Settings, error) {
+	db, err := DB()
+	if err != nil {
+		return model.Settings{}, err
+	}
+	var items []model.Setting
+	if err := db.Find(&items).Error; err != nil {
+		return model.Settings{}, err
+	}
+	result := model.Settings{}
+	for _, item := range items {
+		if item.Key == model.SettingKeyPrivate {
+			_ = json.Unmarshal(item.Value, &result.Private)
+		} else if item.Key == model.SettingKeyPublic {
+			_ = json.Unmarshal(item.Value, &result.Public)
+		}
+	}
+	return result, nil
+}
+
+// SaveSettings 保存 public 和 private 两行配置。
+func SaveSettings(settings model.Settings, now string) (model.Settings, error) {
+	db, err := DB()
+	if err != nil {
+		return settings, err
+	}
+	publicValue, _ := json.Marshal(settings.Public)
+	privateValue, _ := json.Marshal(settings.Private)
+	items := []model.Setting{
+		{Key: model.SettingKeyPublic, Value: publicValue, CreatedAt: now, UpdatedAt: now},
+		{Key: model.SettingKeyPrivate, Value: privateValue, CreatedAt: now, UpdatedAt: now},
+	}
+	err = db.Clauses(clause.OnConflict{
+		Columns:   []clause.Column{{Name: "key"}},
+		DoUpdates: clause.AssignmentColumns([]string{"value", "updated_at"}),
+	}).Create(&items).Error
+	return settings, err
+}