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

web/src/services/api/admin.ts

@@ -81,3 +81,39 @@ export async function saveAdminAsset(token: string, asset: Partial<AdminAsset>)
 export async function deleteAdminAsset(token: string, id: string) {
   return apiDelete<boolean>(`/api/admin/assets/${encodeURIComponent(id)}`, token);
 }
+
+export type AdminModelChannel = {
+  key: string;
+  name: string;
+  baseUrl: string;
+  apiKey: string;
+  models: string[];
+  enabled: boolean;
+  remark: string;
+};
+
+export type AdminPublicSettings = {
+  availableModels: string[];
+  defaultModel: string;
+  defaultImageModel: string;
+  defaultTextModel: string;
+  systemPrompt: string;
+  allowCustomModel: boolean;
+};
+
+export type AdminPrivateSettings = {
+  channels: AdminModelChannel[];
+};
+
+export type AdminSettings = {
+  public: AdminPublicSettings;
+  private: AdminPrivateSettings;
+};
+
+export async function fetchAdminSettings(token: string) {
+  return apiGet<AdminSettings>("/api/admin/settings", undefined, token);
+}
+
+export async function saveAdminSettings(token: string, settings: AdminSettings) {
+  return apiPost<AdminSettings>("/api/admin/settings", settings, token);
+}