docs: clarify wildcard MX requirement for random subdomain (#1036)

Random subdomain mailbox creation only generates addresses; mail delivery
depends on DNS / Cloudflare Email Routing covering *.<base-domain>.
Cloudflare Email Routing does not inherit apex configuration onto
subdomains, so a wildcard `*` MX record on the base domain is required
for random subdomains to actually receive mail.

- Add `[!IMPORTANT]` block in subdomain.md (zh/en) explaining the two
  deliverable paths: DNS-only wildcard MX (recommended for random
  subdomains) vs Cloudflare dashboard "Add subdomain"
- Link to Cloudflare Email Routing — Subdomains official docs from
  worker-vars.md and subdomain.md
- Instruct copying apex MX records to host `*` preserving each record's
  priority/target, instead of hardcoding specific MX targets
- Shorten frontend `randomSubdomainTip` for CreateAccount and Login
  views (6 locales: zh/en/de/es/ja/pt-BR), drop Markdown backticks
  (Vue text interpolation renders them literally), and point users to
  the docs instead of embedding DNS instructions
- Trim overlap between existing `[!NOTE]` and new `[!IMPORTANT]` in
  subdomain.md
- Update CHANGELOG.md / CHANGELOG_EN.md under v1.9.0(main)

Closes #1035
Closes #1026

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

CHANGELOG.md

@@ -11,6 +11,7 @@
 ### Features
 
 - feat: |Frontend| 将邮箱地址凭证弹窗升级为“地址凭证与连接方式”，复用普通用户与 admin 创建邮箱结果弹窗；支持通过 `ENABLE_AGENT_EMAIL_INFO` 展示 AI Agent 接入信息，并通过 `SMTP_IMAP_PROXY_CONFIG` 展示 SMTP/IMAP 客户端连接信息
+- docs: |随机子域名| 在前端“启用随机子域名”提示与 `subdomain` / `worker-vars` 文档（中英）中明确说明：要让 `name@<随机>.abc.com` 真正收到邮件，必须在基础域名 DNS 中为 `*` 子域添加通配 MX 记录，Email Routing 子域不继承父域配置（issue #1035）
 
 ### Bug Fixes