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/day-detail.css

@@ -62,10 +62,10 @@ body.day-detail-sheet-open {
 .day-detail-panel--sheet::before {
   content: "";
   display: block;
-  width: 36px;
+  width: 40px;
   height: 4px;
   margin: 0 auto 8px;
-  background: var(--muted);
+  background: color-mix(in srgb, var(--muted) 80%, var(--text));
   border-radius: 2px;
 }
 
@@ -79,13 +79,19 @@ body.day-detail-sheet-open {
   border: none;
   background: transparent;
   color: var(--muted);
-  font-size: 1.5rem;
   line-height: 1;
   cursor: pointer;
   border-radius: 8px;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
   transition: opacity var(--transition-fast), background-color var(--transition-fast);
 }
 
+.day-detail-close svg {
+  display: block;
+}
+
 .day-detail-close:focus {
   outline: none;
 }