Enhance calendar layout and duty list rendering

- Introduced a sticky header for the calendar to improve navigation.
- Updated the duty list display to highlight today's date with a distinct style.
- Added new CSS styles for better visual presentation of duty days and improved layout consistency.

webapp/style.css

@@ -73,6 +73,19 @@ body {
   text-align: center;
 }
 
+.calendar-sticky {
+  position: sticky;
+  top: 0;
+  z-index: 10;
+  background: var(--bg);
+  padding-bottom: 12px;
+  margin-bottom: 4px;
+}
+
+.calendar-sticky.is-scrolled {
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.25);
+}
+
 .calendar {
   display: grid;
   grid-template-columns: repeat(7, 1fr);
@@ -215,6 +228,26 @@ body {
   margin: 0 0 8px 0;
 }
 
+.duty-list-day {
+  margin-bottom: 16px;
+}
+
+.duty-list-day--today .duty-list-day-title {
+  color: var(--today);
+  font-weight: 700;
+}
+
+.duty-list-day--today .duty-list-day-title::before {
+  content: "";
+  display: inline-block;
+  width: 4px;
+  height: 1em;
+  background: var(--today);
+  border-radius: 2px;
+  margin-right: 8px;
+  vertical-align: middle;
+}
+
 .duty-item {
   display: grid;
   grid-template-columns: 5.5em 1fr;