feat: split schedule view horizontal axis into AM/PM half-day slots

Agent-Logs-Url: https://github.com/pdf114514/CarReservation/sessions/b0f3486c-edf5-4c08-b1e8-161b380aa0a0

Co-authored-by: pdf114514 <57948770+pdf114514@users.noreply.github.com>

frontend/src/components/ScheduleView.module.css

@@ -124,6 +124,47 @@
   z-index: 30;
 }
 
+.dateHeaderGroup {
+  flex-shrink: 0;
+  display: flex;
+  flex-direction: column;
+  align-items: stretch;
+  border-right: 2px solid #d1d5db;
+  background: white;
+  overflow: hidden;
+}
+
+.dateHeaderTop {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 2px;
+  flex: 1;
+  border-bottom: 1px solid #e5e7eb;
+}
+
+.dateHeaderAmPm {
+  display: flex;
+  height: 22px;
+  flex-shrink: 0;
+}
+
+.periodLabel {
+  flex: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 9px;
+  font-weight: 500;
+  color: #9ca3af;
+  letter-spacing: 0.03em;
+}
+
+.periodLabel:first-child {
+  border-right: 1px solid #f0f0f0;
+}
+
 .dateHeader {
   flex-shrink: 0;
   display: flex;
@@ -281,6 +322,10 @@
   background: rgba(26, 86, 219, 0.04);
 }
 
+.cellDayEnd {
+  border-right: 1px solid #d1d5db;
+}
+
 .todayCell {
   background: rgba(59, 130, 246, 0.06);
 }