Fix timezone bug in event creation

Events were appearing 4 hours earlier than selected time due to incorrect
timezone handling in backend. The issue was treating frontend local time
as if it was already in UTC.

- Fix parse_event_datetime() in events.rs to properly convert local time to UTC
- Fix all datetime conversions in series.rs to use Local timezone conversion
- Replace Utc.from_utc_datetime() with proper Local.from_local_datetime()
- Add timezone conversion using with_timezone(&Utc) for accurate UTC storage

Now when user selects 5:00 AM, it correctly stores as UTC equivalent
and displays back at 5:00 AM local time.

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

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

backend/src/handlers/events.rs

@@ -845,7 +845,7 @@ fn parse_event_datetime(
     time_str: &str,
     all_day: bool,
 ) -> Result<chrono::DateTime<chrono::Utc>, String> {
-    use chrono::{NaiveDate, NaiveDateTime, NaiveTime, TimeZone, Utc};
+    use chrono::{Local, NaiveDate, NaiveDateTime, NaiveTime, TimeZone, Utc};
 
     // Parse the date
     let date = NaiveDate::parse_from_str(date_str, "%Y-%m-%d")
@@ -865,7 +865,11 @@ fn parse_event_datetime(
         // Combine date and time
         let datetime = NaiveDateTime::new(date, time);
 
-        // Assume local time and convert to UTC (in a real app, you'd want timezone support)
-        Ok(Utc.from_utc_datetime(&datetime))
+        // Treat the datetime as local time and convert to UTC
+        let local_datetime = Local.from_local_datetime(&datetime)
+            .single()
+            .ok_or_else(|| "Ambiguous local datetime".to_string())?;
+        
+        Ok(local_datetime.with_timezone(&Utc))
     }
 }