fix(loyalty): sweep remaining hardcoded 'en-US' in persona JS files

Follow-up to 06e59f73 which swept non-loyalty modules. The earlier
loyalty fix (dd1f9af8) only touched the shared/ factories; persona-
specific JS files in loyalty's admin/, merchant/, store/, and
storefront/ dirs were missed and still hardcoded 'en-US'.

13 occurrences across 8 files now use I18n.locale:
- admin: loyalty-analytics.js, loyalty-merchant-detail.js,
  loyalty-programs.js
- merchant: loyalty-analytics.js
- store: loyalty-analytics.js, loyalty-terminal.js
- storefront: loyalty-dashboard.js, loyalty-history.js

After this commit grep -rn "'en-US'" --include=*.js across the whole
repo returns nothing. Clearing the deck so the JS-016 rule can ship
at error severity in the next commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/modules/loyalty/static/admin/js/loyalty-programs.js

@@ -325,7 +325,7 @@ function adminLoyaltyPrograms() {
             if (!dateString) return 'N/A';
             try {
                 const date = new Date(dateString);
-                return date.toLocaleDateString('en-US', {
+                return date.toLocaleDateString(I18n.locale, {
                     year: 'numeric',
                     month: 'short',
                     day: 'numeric'
@@ -339,7 +339,7 @@ function adminLoyaltyPrograms() {
         // Format number with thousands separator
         formatNumber(num) {
             if (num === null || num === undefined) return '0';
-            return new Intl.NumberFormat('en-US').format(num);
+            return new Intl.NumberFormat(I18n.locale).format(num);
         }
     };
 }