feat: hide send mail UI when not configured (#827)

- Add isSendMailEnabled and isAnySendMailEnabled functions in common.ts
- Return enableSendMail field in /open_api/settings
- Hide sendmail tab, sendbox tab, and reply button when send mail is not configured
- Check RESEND_TOKEN, SMTP_CONFIG, and SEND_MAIL binding per domain

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/store/index.js

@@ -34,6 +34,7 @@ export const useGlobalState = createGlobalState(
             cfTurnstileSiteKey: '',
             enableWebhook: false,
             isS3Enabled: false,
+            enableSendMail: false,
             showGithub: true,
             disableAdminPasswordCheck: false,
             enableAddressPassword: false,

frontend/src/views/Index.vue

@@ -156,15 +156,15 @@ onMounted(() => {
               </n-button>
             </n-input-group>
           </div>
-          <MailBox :key="mailBoxKey" :showEMailTo="false" :showReply="true" :showSaveS3="openSettings.isS3Enabled"
+          <MailBox :key="mailBoxKey" :showEMailTo="false" :showReply="openSettings.enableSendMail" :showSaveS3="openSettings.isS3Enabled"
             :saveToS3="saveToS3" :enableUserDeleteEmail="openSettings.enableUserDeleteEmail"
             :fetchMailData="fetchMailData" :deleteMail="deleteMail" :showFilterInput="true" />
         </n-tab-pane>
-        <n-tab-pane name="sendbox" :tab="t('sendbox')">
+        <n-tab-pane v-if="openSettings.enableSendMail" name="sendbox" :tab="t('sendbox')">
           <SendBox :fetchMailData="fetchSenboxData" :enableUserDeleteEmail="openSettings.enableUserDeleteEmail"
             :deleteMail="deleteSenboxMail" />
         </n-tab-pane>
-        <n-tab-pane name="sendmail" :tab="t('sendmail')">
+        <n-tab-pane v-if="openSettings.enableSendMail" name="sendmail" :tab="t('sendmail')">
           <SendMail />
         </n-tab-pane>
         <n-tab-pane name="accountSettings" :tab="t('accountSettings')">