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

service/settings.go

@@ -0,0 +1,45 @@
+package service
+
+import (
+	"github.com/basketikun/infinite-canvas/model"
+	"github.com/basketikun/infinite-canvas/repository"
+)
+
+func PublicSettings() (model.PublicSetting, error) {
+	settings, err := repository.GetSettings()
+	return normalizePublicSetting(settings.Public), err
+}
+
+func AdminSettings() (model.Settings, error) {
+	settings, err := repository.GetSettings()
+	return normalizeSettings(settings), err
+}
+
+func SaveSettings(settings model.Settings) (model.Settings, error) {
+	return repository.SaveSettings(normalizeSettings(settings), now())
+}
+
+func normalizeSettings(settings model.Settings) model.Settings {
+	settings.Public = normalizePublicSetting(settings.Public)
+	settings.Private = normalizePrivateSetting(settings.Private)
+	return settings
+}
+
+func normalizePublicSetting(setting model.PublicSetting) model.PublicSetting {
+	if setting.AvailableModels == nil {
+		setting.AvailableModels = []string{}
+	}
+	return setting
+}
+
+func normalizePrivateSetting(setting model.PrivateSetting) model.PrivateSetting {
+	if setting.Channels == nil {
+		setting.Channels = []model.ModelChannel{}
+	}
+	for i := range setting.Channels {
+		if setting.Channels[i].Models == nil {
+			setting.Channels[i].Models = []string{}
+		}
+	}
+	return setting
+}