Enhance month view overlay with preview, auto-scroll, and 1s delay

- Fix backend type comparison in AvailabilityService (int vs string)
- Add durationMinutes to month overlay drop to fix end_time calculation
- Add live preview of dragged appointment in overlay with lane splitting
- Implement horizontal auto-scroll when dragging to overlay edges
- Add 1-second delay before overlay appears for easier date navigation
- Remove redundant drop zone highlight (preview shows position)

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

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

frontend/src/hooks/useAppointments.ts

@@ -136,7 +136,8 @@ export const useUpdateAppointment = () => {
 
       if (updates.serviceId) backendData.service = parseInt(updates.serviceId);
       if (updates.resourceId !== undefined) {
-        backendData.resource = updates.resourceId ? parseInt(updates.resourceId) : null;
+        // Backend expects resource_ids as a list
+        backendData.resource_ids = updates.resourceId ? [parseInt(updates.resourceId)] : [];
       }
       if (updates.startTime) {
         backendData.start_time = updates.startTime.toISOString();