feat: Split datetime fields

- Replace single datetime-local field with 4 separate fields:

- Add helper functions:
  * splitDateTime(): Split datetime string into date and time parts
  * combineDateTime(): Combine date and time into datetime string

- Implement internal state management:
  * startDate, startTime, endDate, endTime internal states
  * Sync effects to update internal states from start/end props
  * Change handlers for each field that combine values and call parent handlers

- Fix test cases:
  * Update EventModal.test.tsx, EventUpdateModal.test.tsx to use new field labels
  * Change from 'Start'/'End' to 'Start Date'/'Start Time'/'End Time'/'End Date'

src/features/Events/EventModal.tsx

@@ -201,6 +201,11 @@ function EventPopover({
     }
 
     if (selectedRange && selectedRange.start && selectedRange.end) {
+      // Auto-check allday if selectedRange is all-day
+      if (selectedRange.allDay) {
+        setAllDay(true);
+      }
+
       // selectedRange gives us the visual time displayed on calendar
       // Use selectedRange.startStr and endStr if available (from FullCalendar)
       if (selectedRange.startStr && selectedRange.endStr) {
@@ -249,6 +254,9 @@ function EventPopover({
 
       if (formattedStart) setStart(formattedStart);
       if (formattedEnd) setEnd(formattedEnd);
+
+      // Default to non-all-day when no selectedRange
+      setAllDay(false);
     }
 
     shouldSyncFromRangeRef.current = false;