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/dayDetail.js

@@ -339,10 +339,14 @@ function ensurePanelInDom() {
     panelEl.hidden = true;
 
     const closeLabel = t(state.lang, "day_detail.close");
+    const closeIcon =
+      '<svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><line x1="18" y1="6" x2="6" y2="18"/><line x1="6" y1="6" x2="18" y2="18"/></svg>';
     panelEl.innerHTML =
       '<button type="button" class="day-detail-close" aria-label="' +
       escapeHtml(closeLabel) +
-      '">×</button><div class="day-detail-body"></div>';
+      '">' +
+      closeIcon +
+      '</button><div class="day-detail-body"></div>';
 
     const closeBtn = panelEl.querySelector(".day-detail-close");
     if (closeBtn) {