fix(rewards): localize campaign date (#29808)

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## **Related issues**

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: swaps a hardcoded month-name formatter for cached
`Intl.DateTimeFormat` using the app locale, plus a small Jest mock
adjustment; behavior changes are limited to date label rendering.
> 
> **Overview**
> Campaign tile date labels are now **localized** by formatting
month/day via `getIntlDateTimeFormatter` using `I18n.locale`, replacing
the prior hardcoded English month list.
> 
> Tests were updated to mock the `i18n` module’s default export
(`locale`) so the new locale-aware formatter can run under Jest.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
38c82e4. Bugbot is set up for automated
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<!-- /CURSOR_SUMMARY -->

Author: sophieqgu

app/components/UI/Rewards/components/Campaigns/CampaignTile.utils.test.ts

@@ -23,6 +23,8 @@ jest.mock('@metamask/design-system-react-native', () => ({
 }));
 
 jest.mock('../../../../../../locales/i18n', () => ({
+  __esModule: true,
+  default: { locale: 'en-US' },
   strings: jest.fn((key: string, params?: Record<string, string>) =>
     params ? `${key}:${JSON.stringify(params)}` : key,
   ),

app/components/UI/Rewards/components/Campaigns/CampaignTile.utils.ts

@@ -4,7 +4,8 @@ import {
   type CampaignDto,
   type CampaignStatus,
 } from '../../../../../core/Engine/controllers/rewards-controller/types';
-import { strings } from '../../../../../../locales/i18n';
+import I18n, { strings } from '../../../../../../locales/i18n';
+import { getIntlDateTimeFormatter } from '../../../../../util/intl';
 
 /**
  * Set of campaign types that have full UI support (details view, opt-in, etc.)
@@ -53,32 +54,18 @@ export function getCampaignStatus(campaign: CampaignDto): CampaignStatus {
   return 'complete';
 }
 
-const MONTHS = [
-  'January',
-  'February',
-  'March',
-  'April',
-  'May',
-  'June',
-  'July',
-  'August',
-  'September',
-  'October',
-  'November',
-  'December',
-];
-
 /**
- * Formats a date for display in campaign tiles.
+ * Formats a date for display in campaign tiles (localized month and day).
  *
  * @param date - The date to format
- * @returns Formatted date string (e.g., "March 15")
+ * @param locale - BCP 47 locale; defaults to the app locale
+ * @returns Formatted date string (e.g., "March 15" in en-US)
  */
-function formatCampaignDate(date: Date): string {
-  const month = MONTHS[date.getMonth()];
-  const day = date.getDate();
-
-  return `${month} ${day}`;
+function formatCampaignDate(date: Date, locale: string = I18n.locale): string {
+  return getIntlDateTimeFormatter(locale, {
+    month: 'long',
+    day: 'numeric',
+  }).format(date);
 }
 
 /**