Fix all-day event date display bug: events created 9/4-9/6 now show correctly instead of 9/3-9/5

- Backend: Store all-day events at noon UTC instead of midnight to avoid timezone boundary issues
- Backend: Remove local timezone conversion for all-day events in series handler
- Frontend: Skip timezone conversion when extracting dates from all-day events for display
- Frontend: Extract dates directly from UTC for all-day events in event_spans_date function

The issue was that timezone conversion of UTC midnight could shift dates backward in western timezones.
Now all-day events use noon UTC storage and pure date extraction without timezone conversion.

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

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

frontend/src/components/week_view.rs

@@ -1320,11 +1320,23 @@ fn calculate_event_layout(events: &[VEvent], date: NaiveDate, time_increment: u3
 
 // Check if an all-day event spans the given date
 fn event_spans_date(event: &VEvent, date: NaiveDate) -> bool {
-    let start_date = event.dtstart.with_timezone(&Local).date_naive();
+    let start_date = if event.all_day {
+        // For all-day events, extract date directly from UTC without timezone conversion
+        // since all-day events are stored at noon UTC to avoid timezone boundary issues
+        event.dtstart.date_naive()
+    } else {
+        event.dtstart.with_timezone(&Local).date_naive()
+    };
+    
     let end_date = if let Some(dtend) = event.dtend {
-        // For all-day events, dtend is often set to the day after the last day
-        // So we need to subtract a day to get the actual last day of the event
-        dtend.with_timezone(&Local).date_naive() - chrono::Duration::days(1)
+        if event.all_day {
+            // For all-day events, dtend is set to the day after the last day (RFC 5545)
+            // Extract date directly from UTC and subtract a day to get actual last day
+            dtend.date_naive() - chrono::Duration::days(1)
+        } else {
+            // For timed events, use timezone conversion
+            dtend.with_timezone(&Local).date_naive()
+        }
     } else {
         // Single day event
         start_date