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>

vitepress-docs/docs/en/guide/worker-vars.md

@@ -51,6 +51,15 @@
 > `RANDOM_SUBDOMAIN_DOMAINS` only controls automatic random subdomain generation during mailbox
 > creation. It does not create Cloudflare-side subdomain routing for you.
 >
+> To actually receive mail on addresses like `name@<random>.abc.com`, **you must add a wildcard
+> `*` MX record under the base domain in DNS** by copying the apex's existing MX records to
+> host `*` (preserving each record's priority and target). Cloudflare Email Routing does not
+> inherit the apex configuration onto subdomains — see the
+> [Cloudflare Email Routing — Subdomains](https://developers.cloudflare.com/email-routing/setup/subdomains/)
+> docs,
+> [#1035](https://github.com/dreamhunter2333/cloudflare_temp_email/issues/1035) and
+> [Configure Subdomain Email](/en/guide/feature/subdomain).
+>
 > Subdomain addresses are usually best used for receiving only; for sending, prefer the main
 > domain.
 >