feat(邮件通知): 重构邮件通知系统并增加配置选项

- 移除 Cloudflare Email 绑定，改为支持外部邮件网关
- 新增邮件通知全局开关及管理员/用户通知独立配置
- 在管理后台添加邮件通知配置界面
- 优化邮件发送逻辑，增加错误处理和日志记录
- 更新相关文档说明

cwd-comments-admin/src/api/admin.ts

@@ -40,6 +40,12 @@ export type CommentSettingsResponse = {
 	adminEnabled: boolean;
 };
 
+export type EmailNotifySettingsResponse = {
+	globalEnabled: boolean;
+	adminEnabled: boolean;
+	userEnabled: boolean;
+};
+
 export async function loginAdmin(name: string, password: string): Promise<string> {
 	const res = await post<AdminLoginResponse>('/admin/login', { name, password });
 	const key = res.data.key;
@@ -71,6 +77,18 @@ export function saveAdminEmail(email: string): Promise<{ message: string }> {
 	return put<{ message: string }>('/admin/settings/email', { email });
 }
 
+export function fetchEmailNotifySettings(): Promise<EmailNotifySettingsResponse> {
+	return get<EmailNotifySettingsResponse>('/admin/settings/email-notify');
+}
+
+export function saveEmailNotifySettings(data: {
+	globalEnabled?: boolean;
+	adminEnabled?: boolean;
+	userEnabled?: boolean;
+}): Promise<{ message: string }> {
+	return put<{ message: string }>('/admin/settings/email-notify', data);
+}
+
 export function fetchCommentSettings(): Promise<CommentSettingsResponse> {
 	return get<CommentSettingsResponse>('/admin/settings/comments');
 }