fix: invalid language tag in heatmap component in reports page (#12952)

## Description

This PR fixes a `RangeError: Invalid language tag` that occurs in the
Heatmap report when using locales with underscores (e.g., `pt_BR`,
`zh_TW`).

The issue was caused by passing the raw locale string from `vue-i18n`
(which uses underscores for some regions) directly to
`Intl.DateTimeFormat`. The `Intl` API expects BCP 47 language tags which
use hyphens (e.g., `pt-BR`).

This change sanitizes the locale string by replacing underscores with
hyphens before creating the `DateTimeFormat` instance.

Fixes #12951

app/javascript/dashboard/routes/dashboard/settings/reports/components/heatmaps/HeatmapDateRangeSelector.vue

@@ -52,11 +52,12 @@ const dayMenuItemConfigs = computed(() => [
   },
 ]);
 
-const resolvedLocale = computed(
-  () =>
+const resolvedLocale = computed(() => {
+  const currentLocale =
     locale.value ||
-    (typeof navigator !== 'undefined' ? navigator.language : 'en')
-);
+    (typeof navigator !== 'undefined' ? navigator.language : 'en');
+  return currentLocale.replace('_', '-');
+});
 
 const monthFormatter = computed(
   () =>