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/css/hints.css

@@ -0,0 +1,70 @@
+/* === Hints (tooltips) */
+.calendar-event-hint {
+  position: fixed;
+  z-index: 1000;
+  width: max-content;
+  max-width: min(98vw, 900px);
+  min-width: 0;
+  padding: 8px 12px;
+  background: var(--surface);
+  color: var(--text);
+  border-radius: 8px;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.4);
+  font-size: 0.85rem;
+  line-height: 1.4;
+  white-space: pre;
+  overflow: visible;
+  transform: translateY(-100%);
+  transition: opacity 0.15s ease-out, transform 0.15s ease-out;
+}
+
+.calendar-event-hint:not(.calendar-event-hint--visible) {
+  opacity: 0;
+}
+
+.calendar-event-hint.calendar-event-hint--visible {
+  opacity: 1;
+}
+
+.calendar-event-hint.below {
+  transform: none;
+}
+
+.calendar-event-hint-title {
+  margin-bottom: 4px;
+  font-weight: 600;
+}
+
+.calendar-event-hint-rows {
+  display: table;
+  width: min-content;
+  table-layout: auto;
+  border-collapse: separate;
+  border-spacing: 0 2px;
+}
+
+.calendar-event-hint-row {
+  display: table-row;
+  white-space: nowrap;
+}
+
+.calendar-event-hint-row .calendar-event-hint-time {
+  display: table-cell;
+  white-space: nowrap;
+  width: 1%;
+  vertical-align: top;
+  text-align: right;
+  padding-right: 0.15em;
+}
+
+.calendar-event-hint-row .calendar-event-hint-sep {
+  display: table-cell;
+  width: 1em;
+  vertical-align: top;
+  padding-right: 0.1em;
+}
+
+.calendar-event-hint-row .calendar-event-hint-name {
+  display: table-cell;
+  white-space: nowrap !important;
+}