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/css/hints.css

@@ -3,7 +3,7 @@
   position: fixed;
   z-index: 1000;
   width: max-content;
-  max-width: min(98vw, 900px);
+  max-width: min(98vw, 380px);
   min-width: 0;
   padding: 8px 12px;
   background: var(--surface);