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/base.css

@@ -89,6 +89,15 @@ body {
   padding-bottom: env(safe-area-inset-bottom, 12px);
 }
 
+/* Muted text (e.g. empty duty list message). */
+.muted {
+  color: var(--muted);
+}
+
 [data-theme="light"] .container {
   border-radius: 12px;
 }
+
+[data-theme="dark"] .container {
+  border-radius: 12px;
+}