feat: enhance CI workflow and update webapp styles

- Added Node.js setup and webapp testing steps to the CI workflow for improved integration.
- Updated HTML to link multiple CSS files for better modularity and organization of styles.
- Removed deprecated `style.css` and introduced new CSS files for base styles, calendar, day detail, hints, markers, states, and duty list to enhance maintainability and readability.
- Implemented new styles for improved presentation of duty information and user interactions.
- Added unit tests for new API functions and contact link rendering to ensure functionality and reliability.

webapp/js/calendar.js

@@ -2,7 +2,7 @@
  * Calendar grid and events-by-date mapping.
  */
 
-import { calendarEl, monthTitleEl, state } from "./dom.js";
+import { getCalendarEl, getMonthTitleEl, state } from "./dom.js";
 import { monthName, t } from "./i18n.js";
 import { escapeHtml } from "./utils.js";
 import {
@@ -71,6 +71,8 @@ export function renderCalendar(
   dutiesByDateMap,
   calendarEventsByDateMap
 ) {
+  const calendarEl = getCalendarEl();
+  const monthTitleEl = getMonthTitleEl();
   if (!calendarEl || !monthTitleEl) return;
   const first = firstDayOfMonth(new Date(year, month, 1));
   const last = lastDayOfMonth(new Date(year, month, 1));