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