Fix all-day recurring events RFC-5545 compliance

- Set all_day flag properly when creating VEvent in series handler
- Improve all-day event detection using VALUE=DATE parameter
- Add RFC-5545 compliance for exclusive end dates (backend adds 1 day)
- Fix end date display in event modal (frontend subtracts 1 day for display)
- Fix recurring all-day event expansion to maintain proper end date pattern

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

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

backend/src/handlers/series.rs

@@ -175,6 +175,7 @@ pub async fn create_event_series(
     // Create the VEvent for the series
     let mut event = VEvent::new(uid.clone(), start_datetime);
     event.dtend = Some(end_datetime);
+    event.all_day = request.all_day; // Set the all_day flag properly
     event.summary = if request.title.trim().is_empty() {
         None
     } else {