feat(sub): show the announcement on the subscription info page

The subAnnounce setting was only emitted as a base64 Announce response header, which most client apps ignore and browsers never see. Pass it into the sub page view-model and render it as an info alert at the top of the card; custom themes get the announce key for free.

Closes #5276

frontend/src/env.d.ts

@@ -18,6 +18,7 @@ interface SubPageData {
   links?: string[];
   emails?: string[];
   datepicker?: 'gregorian' | 'jalalian';
+  announce?: string;
   downloadByte?: string | number;
   uploadByte?: string | number;
   usedByte?: string | number;

frontend/src/pages/sub/SubPage.tsx

@@ -1,6 +1,7 @@
 import { Fragment, useCallback, useEffect, useMemo, useState } from 'react';
 import { useTranslation } from 'react-i18next';
 import {
+  Alert,
   Button,
   Card,
   Col,
@@ -61,6 +62,7 @@ const links: string[] = Array.isArray(subData.links) ? subData.links : [];
 const linkEmails: string[] = Array.isArray(subData.emails) ? subData.emails : [];
 const subEmail = [...new Set(linkEmails.filter(Boolean))].join(', ');
 const datepicker = subData.datepicker || 'gregorian';
+const announce = subData.announce || '';
 
 const isUnlimited = totalByte <= 0 && expireMs === 0;
 const isActive = (() => {
@@ -283,6 +285,9 @@ export default function SubPage() {
           <Row justify="center">
             <Col xs={24} sm={22} md={18} lg={14} xl={12}>
               <Card hoverable className="subscription-card" title={cardTitle} extra={cardExtra}>
+                {announce && (
+                  <Alert type="info" showIcon message={announce} style={{ marginBottom: 16 }} />
+                )}
                 <Descriptions
                   bordered
                   column={1}

internal/sub/controller.go

@@ -281,6 +281,7 @@ func (a *SUBController) serveSubPage(c *gin.Context, basePath string, page PageD
 		"links":         page.Result,
 		"emails":        page.Emails,
 		"datepicker":    datepicker,
+		"announce":      a.subAnnounce,
 	}
 
 	// When an admin has configured a custom subscription theme, render it