Refactors header and scroll management logic

Updates ScrollManager to dynamically sync header spacer height using ResizeObserver

Removes explicit spacer animation from HeaderDrawerManager
Simplifies header and scroll interaction by moving height synchronization logic to ScrollManager

wwwroot/css/calendar-v2.css

@@ -5,6 +5,7 @@
   --day-column-min-width: 200px;
   --day-start-hour: 6;
   --day-end-hour: 18;
+  --header-height: 70px;
   --color-border: #e0e0e0;
   --color-surface: #fff;
   --color-text-secondary: #666;
@@ -81,12 +82,16 @@ swp-time-axis {
   grid-column: 1;
   grid-row: 1 / 3;
   display: grid;
-  grid-template-rows: subgrid;
+  grid-template-rows: auto 1fr;
   border-right: 1px solid var(--color-border);
   background: var(--color-surface);
   overflow: hidden;
 }
 
+swp-header-spacer {
+  border-bottom: 1px solid var(--color-border);
+}
+
 swp-header-drawer {
   display: block;
   height: 0;
@@ -96,9 +101,9 @@ swp-header-drawer {
 }
 
 swp-time-axis-content {
-  display: grid;
-  grid-auto-rows: var(--hour-height);
-  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+  position: relative;
 }
 
 swp-hour-marker {
@@ -118,6 +123,10 @@ swp-hour-marker {
     height: 1px;
     background: var(--color-hour-line);
   }
+
+  &:first-child::after {
+    display: none;
+  }
 }
 
 /* Grid container */