Improves date header localization

Adds locale support to the date header renderer and date service.

This change ensures that the day names are displayed correctly based on the user's configured locale.

The date header now uses the configured locale to display the day name in the correct language.
The day name is now displayed in uppercase.

Additionally, styles the date header to highlight today's date.

src/core/CalendarConfig.ts

@@ -546,6 +546,13 @@ export class CalendarConfig {
     return this.timeFormatConfig.timezone;
   }
 
+  /**
+   * Get configured locale
+   */
+  getLocale(): string {
+    return this.timeFormatConfig.locale;
+  }
+
   /**
    * Check if using 24-hour format
    */