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/ui.test.js

@@ -93,7 +93,8 @@ describe("ui", () => {
     it("sets error text and shows error element", () => {
       showError("Network error");
       const errorEl = document.getElementById("error");
-      expect(errorEl?.textContent).toBe("Network error");
+      const textEl = errorEl?.querySelector(".error-text");
+      expect(textEl?.textContent).toBe("Network error");
       expect(errorEl?.hidden).toBe(false);
     });