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/main.js

@@ -189,7 +189,7 @@ async function loadMonth() {
       }
       return;
     }
-    showError(e.message || t(state.lang, "error_generic"));
+    showError(e.message || t(state.lang, "error_generic"), loadMonth);
     setNavEnabled(true);
     return;
   }