feat: enhance UI components and error handling

- Updated HTML structure for navigation buttons in the calendar, adding SVG icons for improved visual clarity.
- Introduced a new muted text style in CSS for better presentation of empty duty list messages.
- Enhanced calendar CSS for navigation buttons and day indicators, improving layout and responsiveness.
- Improved error handling in the UI by adding retry functionality to the error display, allowing users to retry actions directly from the error message.
- Updated internationalization messages to include a retry option for error handling.
- Added unit tests to verify the new error handling behavior and UI updates.

webapp/js/i18n.js

@@ -13,6 +13,7 @@ export const MESSAGES = {
     error_load_failed: "Load failed",
     error_network: "Could not load data. Check your connection.",
     error_generic: "Could not load data.",
+    "error.retry": "Retry",
     "nav.prev_month": "Previous month",
     "nav.next_month": "Next month",
     "weekdays.mon": "Mon",
@@ -68,6 +69,7 @@ export const MESSAGES = {
     error_load_failed: "Ошибка загрузки",
     error_network: "Не удалось загрузить данные. Проверьте интернет.",
     error_generic: "Не удалось загрузить данные.",
+    "error.retry": "Повторить",
     "nav.prev_month": "Предыдущий месяц",
     "nav.next_month": "Следующий месяц",
     "weekdays.mon": "Пн",