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>

frontend/src/i18n/locales/source/de.ts

@@ -577,8 +577,8 @@ export const deMessages = {
   "views.Admin.webhookSettings": "Webhook-Einstellungen",
   "views.admin.AiExtractSettings.disabledTip": "Wenn deaktiviert, verarbeitet die KI-Extraktion alle E-Mail-Adressen",
   "views.admin.AiExtractSettings.enableAllowListTip": "Wenn aktiviert, verarbeitet die KI-Extraktion nur E-Mails an Adressen auf der Freigabeliste",
-  "views.admin.CreateAccount.randomSubdomainTip": "Wenn aktiviert, verwendet die erstellte Adresse eine zufällige Subdomain. Subdomain-Adressen werden nur für den Empfang empfohlen.",
-  "views.common.Login.randomSubdomainTip": "Wenn aktiviert, verwendet die erstellte Adresse eine zufällige Subdomain. Subdomain-Adressen werden nur für den Empfang empfohlen.",
+  "views.admin.CreateAccount.randomSubdomainTip": "Wenn aktiviert, verwendet die erstellte Adresse eine zufällige Subdomain. Nur für den Empfang empfohlen. Erfordert einen Wildcard-MX-DNS-Eintrag auf der Basisdomain — siehe die Dokumentation zu zufälligen Subdomains.",
+  "views.common.Login.randomSubdomainTip": "Wenn aktiviert, verwendet die erstellte Adresse eine zufällige Subdomain. Nur für den Empfang empfohlen. Erfordert einen Wildcard-MX-DNS-Eintrag auf der Basisdomain — siehe die Dokumentation zu zufälligen Subdomains.",
   "views.admin.AiExtractSettings.allowListTip": "Der Platzhalter * passt auf beliebige Zeichen; z. B. passt *{'@'}example.com auf alle Adressen der Domain example.com",
   "views.Admin.workerconfig": "Worker-Konfiguration",
   "views.admin.AccountSettings.create_address_subdomain_match_env_locked": "Die Worker-Umgebungsvariable ENABLE_CREATE_ADDRESS_SUBDOMAIN_MATCH ist derzeit false. Der gespeicherte Admin-Schalter kann geändert werden, wird aber erst wirksam, wenn die Umgebungsvariable aktiviert oder entfernt wird.",

frontend/src/i18n/locales/source/es.ts

@@ -577,8 +577,8 @@ export const esMessages = {
   "views.Admin.webhookSettings": "Configuración de webhook",
   "views.admin.AiExtractSettings.disabledTip": "Si está desactivado, la extracción IA procesará todas las direcciones",
   "views.admin.AiExtractSettings.enableAllowListTip": "Si está activado, la extracción IA solo procesará correos enviados a direcciones permitidas",
-  "views.admin.CreateAccount.randomSubdomainTip": "Si está activado, la dirección creada usará un subdominio aleatorio. Se recomienda usarlo solo para recibir.",
-  "views.common.Login.randomSubdomainTip": "Si está activado, la dirección creada usará un subdominio aleatorio. Se recomienda usarlo solo para recibir.",
+  "views.admin.CreateAccount.randomSubdomainTip": "Si está activado, la dirección creada usará un subdominio aleatorio. Recomendado solo para recibir. Requiere un registro MX comodín en el DNS del dominio base — consulta la documentación de subdominios aleatorios.",
+  "views.common.Login.randomSubdomainTip": "Si está activado, la dirección creada usará un subdominio aleatorio. Recomendado solo para recibir. Requiere un registro MX comodín en el DNS del dominio base — consulta la documentación de subdominios aleatorios.",
   "views.admin.AiExtractSettings.allowListTip": "El comodín * coincide con cualquier carácter; p. ej., *{'@'}example.com coincide con todas las direcciones del dominio example.com",
   "views.Admin.workerconfig": "Configuración del Worker",
   "views.admin.AccountSettings.create_address_subdomain_match_env_locked": "La variable ENABLE_CREATE_ADDRESS_SUBDOMAIN_MATCH está en false. Puedes guardar el interruptor, pero no tendrá efecto hasta habilitar o quitar la variable.",

frontend/src/i18n/locales/source/ja.ts

@@ -577,8 +577,8 @@ export const jaMessages = {
   "views.Admin.webhookSettings": "Webhook設定",
   "views.admin.AiExtractSettings.disabledTip": "無効時は AI 抽出がすべてのメールアドレスを処理します",
   "views.admin.AiExtractSettings.enableAllowListTip": "有効時は AI 抽出は許可リストのアドレス宛メールのみ処理します",
-  "views.admin.CreateAccount.randomSubdomainTip": "有効時は作成されるアドレスがランダムなサブドメインを使用します。サブドメインアドレスは受信専用として推奨されます。",
-  "views.common.Login.randomSubdomainTip": "有効時は作成されるアドレスがランダムなサブドメインを使用します。サブドメインアドレスは受信専用として推奨されます。",
+  "views.admin.CreateAccount.randomSubdomainTip": "有効時は作成されるアドレスがランダムなサブドメインを使用します。受信専用として推奨されます。ベースドメインの DNS にワイルドカード MX レコードの設定が必要です — ランダムサブドメインのドキュメントを参照してください。",
+  "views.common.Login.randomSubdomainTip": "有効時は作成されるアドレスがランダムなサブドメインを使用します。受信専用として推奨されます。ベースドメインの DNS にワイルドカード MX レコードの設定が必要です — ランダムサブドメインのドキュメントを参照してください。",
   "views.admin.AiExtractSettings.allowListTip": "ワイルドカード * は任意の文字に一致します。例: *{'@'}example.com は example.com ドメイン配下のすべてのアドレスに一致します",
   "views.Admin.workerconfig": "Worker設定",
   "views.admin.AccountSettings.create_address_subdomain_match_env_locked": "Worker 環境変数 ENABLE_CREATE_ADDRESS_SUBDOMAIN_MATCH は現在 false です。管理画面のスイッチは保存できますが、env を有効化または削除するまで反映されません。",

frontend/src/i18n/locales/source/ptBR.ts

@@ -577,8 +577,8 @@ export const ptBRMessages = {
   "views.Admin.webhookSettings": "Configurações de webhook",
   "views.admin.AiExtractSettings.disabledTip": "Quando desativado, a extração por IA processará todos os endereços",
   "views.admin.AiExtractSettings.enableAllowListTip": "Quando ativado, a extração por IA só processará e-mails enviados aos endereços permitidos",
-  "views.admin.CreateAccount.randomSubdomainTip": "Quando ativado, o endereço criado usará um subdomínio aleatório. Endereços com subdomínio são recomendados apenas para recebimento.",
-  "views.common.Login.randomSubdomainTip": "Quando ativado, o endereço criado usará um subdomínio aleatório. Endereços com subdomínio são recomendados apenas para recebimento.",
+  "views.admin.CreateAccount.randomSubdomainTip": "Quando ativado, o endereço criado usará um subdomínio aleatório. Recomendado apenas para recebimento. Requer um registro MX curinga no DNS do domínio base — consulte a documentação de subdomínios aleatórios.",
+  "views.common.Login.randomSubdomainTip": "Quando ativado, o endereço criado usará um subdomínio aleatório. Recomendado apenas para recebimento. Requer um registro MX curinga no DNS do domínio base — consulte a documentação de subdomínios aleatórios.",
   "views.admin.AiExtractSettings.allowListTip": "O curinga * corresponde a quaisquer caracteres; ex.: *{'@'}example.com corresponde a todos os endereços do domínio example.com",
   "views.Admin.workerconfig": "Configuração do Worker",
   "views.admin.AccountSettings.create_address_subdomain_match_env_locked": "A variável ENABLE_CREATE_ADDRESS_SUBDOMAIN_MATCH está em false. O botão salvo pode ser alterado, mas só terá efeito quando a variável for ativada ou removida.",

frontend/src/i18n/message-registry.ts

@@ -2001,8 +2001,8 @@ export const MESSAGE_REGISTRY = {
       "zh": "打开即可自动登录邮箱的链接"
     },
     "randomSubdomainTip": {
-      "en": "When enabled, the created address will use a random subdomain. Subdomain addresses are recommended for receiving only.",
-      "zh": "启用后，创建出来的地址会自动挂在随机子域名下。子域名地址更建议仅用于收件。"
+      "en": "When enabled, the created address will use a random subdomain. Recommended for receiving only. Requires a wildcard MX DNS record on the base domain — see the random subdomain docs.",
+      "zh": "启用后，创建出来的地址会自动挂在随机子域名下，建议仅用于收件。需要在基础域名 DNS 中配置通配 MX 记录，详见随机子域名文档。"
     },
     "successTip": {
       "en": "Success Created",
@@ -2523,8 +2523,8 @@ export const MESSAGE_REGISTRY = {
       "zh": "请\"登录\"或点击 \"注册新邮箱\" 按钮来获取一个新的邮箱地址"
     },
     "randomSubdomainTip": {
-      "en": "When enabled, the created address will use a random subdomain. Subdomain addresses are recommended for receiving only.",
-      "zh": "启用后，创建出来的地址会自动挂在随机子域名下。子域名地址更建议仅用于收件。"
+      "en": "When enabled, the created address will use a random subdomain. Recommended for receiving only. Requires a wildcard MX DNS record on the base domain — see the random subdomain docs.",
+      "zh": "启用后，创建出来的地址会自动挂在随机子域名下，建议仅用于收件。需要在基础域名 DNS 中配置通配 MX 记录，详见随机子域名文档。"
     }
   },
   "views.admin.Webhook": {