feat(mail-operations): 补充邮箱服务 UI 入口说明 v1.0.4 (#30)

## Summary / 摘要

- Add "How to Access the Email Service" section to `SKILL.md` and
`SKILL.zh-CN.md`, guiding users to navigate via Resource Explorer →
Emails card
- 在中英文技能文件中新增「如何进入邮箱服务」章节，说明通过资源管理器→邮箱卡片进入邮箱管理界面

## Changes / 变更

- `skills/mail-operations/SKILL.md`: add "How to Access the Email
Service" section, version 1.0.3 → 1.0.4
- `skills/mail-operations/SKILL.zh-CN.md`: add「如何进入邮箱服务」章节
- `manifest.json`: version 1.2.5 → 1.2.6

manifest.json

@@ -1,6 +1,6 @@
 {
   "name": "DesireCore Official Market",
-  "version": "1.2.5",
+  "version": "1.2.6",
   "schemaVersion": "1.1.0",
   "supportedLocales": ["zh-CN", "en-US"],
   "defaultLocale": "en-US",