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/duty-list.css

@@ -41,7 +41,12 @@
   top: 0;
   bottom: 0;
   width: 2px;
-  background: var(--muted);
+  background: linear-gradient(
+    to bottom,
+    var(--muted) 0%,
+    var(--muted) 85%,
+    color-mix(in srgb, var(--muted) 40%, transparent) 100%
+  );
   pointer-events: none;
 }
 
@@ -185,7 +190,7 @@
 }
 
 .duty-flip-inner {
-  transition: transform 0.4s;
+  transition: transform 0.3s;
   transform-style: preserve-3d;
   position: relative;
   min-height: 0;
@@ -253,6 +258,7 @@
   border-radius: 8px;
   background: var(--surface);
   border-left: 3px solid var(--duty);
+  box-shadow: 0 1px 3px color-mix(in srgb, var(--text) 8%, transparent);
 }
 
 .duty-item--unavailable {