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>

backend/src/handlers/events.rs

@@ -852,10 +852,11 @@ fn parse_event_datetime(
         .map_err(|_| format!("Invalid date format: {}. Expected YYYY-MM-DD", date_str))?;
 
     if all_day {
-        // For all-day events, use midnight UTC
+        // For all-day events, use noon UTC to avoid timezone boundary issues
+        // This ensures the date remains correct when converted to any local timezone
         let datetime = date
-            .and_hms_opt(0, 0, 0)
-            .ok_or_else(|| "Failed to create midnight datetime".to_string())?;
+            .and_hms_opt(12, 0, 0)
+            .ok_or_else(|| "Failed to create noon datetime".to_string())?;
         Ok(Utc.from_utc_datetime(&datetime))
     } else {
         // Parse the time

backend/src/handlers/series.rs

@@ -397,8 +397,9 @@ pub async fn update_event_series(
     };
 
     let (start_datetime, end_datetime) = if request.all_day {
+        // For all-day events, use noon UTC to avoid timezone boundary issues
         let start_dt = start_date
-            .and_hms_opt(0, 0, 0)
+            .and_hms_opt(12, 0, 0)
             .ok_or_else(|| ApiError::BadRequest("Invalid start date".to_string()))?;
 
         // For all-day events, also preserve the original date pattern
@@ -414,20 +415,13 @@ pub async fn update_event_series(
         };
 
         let end_dt = end_date
-            .and_hms_opt(23, 59, 59)
+            .and_hms_opt(12, 0, 0)
             .ok_or_else(|| ApiError::BadRequest("Invalid end date".to_string()))?;
 
-        // Convert from local time to UTC
-        let start_local = chrono::Local.from_local_datetime(&start_dt)
-            .single()
-            .ok_or_else(|| ApiError::BadRequest("Ambiguous start datetime".to_string()))?;
-        let end_local = chrono::Local.from_local_datetime(&end_dt)
-            .single()
-            .ok_or_else(|| ApiError::BadRequest("Ambiguous end datetime".to_string()))?;
-        
+        // For all-day events, use UTC directly (no local conversion needed)
         (
-            start_local.with_timezone(&chrono::Utc),
-            end_local.with_timezone(&chrono::Utc),
+            chrono::Utc.from_utc_datetime(&start_dt),
+            chrono::Utc.from_utc_datetime(&end_dt),
         )
     } else {
         let start_time = if !request.start_time.is_empty() {