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/markers.css

@@ -0,0 +1,45 @@
+/* === Markers (duty / unavailable / vacation) */
+.duty-marker,
+.unavailable-marker,
+.vacation-marker {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 11px;
+  height: 11px;
+  padding: 0;
+  border: none;
+  font-size: 0.55rem;
+  font-weight: 700;
+  border-radius: 50%;
+  flex-shrink: 0;
+  cursor: pointer;
+  transition: box-shadow var(--transition-fast) ease-out;
+}
+
+.duty-marker {
+  color: var(--duty);
+  background: color-mix(in srgb, var(--duty) 25%, transparent);
+}
+
+.unavailable-marker {
+  color: var(--unavailable);
+  background: color-mix(in srgb, var(--unavailable) 25%, transparent);
+}
+
+.vacation-marker {
+  color: var(--vacation);
+  background: color-mix(in srgb, var(--vacation) 25%, transparent);
+}
+
+.duty-marker.calendar-marker-active {
+  box-shadow: 0 0 0 2px var(--duty);
+}
+
+.unavailable-marker.calendar-marker-active {
+  box-shadow: 0 0 0 2px var(--unavailable);
+}
+
+.vacation-marker.calendar-marker-active {
+  box-shadow: 0 0 0 2px var(--vacation);
+}