Fix all-day events: validation and proper header positioning

Backend fixes:
- Fix all-day event creation validation error
- Allow same start/end date for all-day events (single-day events)
- Maintain strict validation for timed events (end must be after start)

Frontend improvements:
- Move all-day events from time grid to day headers
- Add dedicated all-day events container that stacks vertically
- Filter all-day events out of main time-based events area
- Add proper CSS styling for all-day event display and interaction
- Maintain event click handling and color themes

All-day events now appear in the correct location at the top of each
day column and properly stack when multiple events exist.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/styles.css

@@ -650,11 +650,14 @@ body {
 }
 
 .week-day-header {
-    padding: 1rem;
+    padding: 0.5rem;
     text-align: center;
     border-right: 1px solid var(--time-label-border, #e9ecef);
     background: var(--weekday-header-bg, #f8f9fa);
     color: var(--weekday-header-text, inherit);
+    min-height: 70px; /* Ensure space for all-day events */
+    display: flex;
+    flex-direction: column;
 }
 
 .week-day-header.today {
@@ -680,6 +683,45 @@ body {
     color: var(--calendar-today-text, #1976d2);
 }
 
+/* All-day events in header */
+.day-header-content {
+    flex-shrink: 0;
+}
+
+.all-day-events {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+    margin-top: 0.5rem;
+    min-height: 0;
+}
+
+.all-day-event {
+    background: #3B82F6;
+    color: white;
+    border-radius: 4px;
+    padding: 2px 6px;
+    font-size: 0.75rem;
+    text-align: left;
+    cursor: pointer;
+    border: 1px solid rgba(255,255,255,0.2);
+    min-height: 18px;
+    display: flex;
+    align-items: center;
+}
+
+.all-day-event:hover {
+    filter: brightness(1.1);
+}
+
+.all-day-event-title {
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+    flex: 1;
+}
+
 /* Week Content */
 .week-content {
     flex: 1;