Fix event creation timezone handling to prevent time offset issues

- Convert local datetime to UTC before sending to backend for non-all-day events
- Keep all-day events unchanged (no timezone conversion needed)
- Add proper timezone conversion using chrono::Local and chrono::Utc
- Include fallback handling if timezone conversion fails
- Add debug logging for timezone conversion issues

This resolves the issue where events appeared 4 hours earlier than expected
due to frontend sending local time but backend treating it as UTC time.

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

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

frontend/src/components/event_form/types.rs

@@ -152,13 +152,50 @@ impl EventCreationData {
         Option<u32>, // recurrence_count
         Option<String>, // recurrence_until
     ) {
+        use chrono::{Local, TimeZone};
+        
+        // Convert local date/time to UTC for backend
+        let (utc_start_date, utc_start_time, utc_end_date, utc_end_time) = if self.all_day {
+            // For all-day events, just use the dates as-is (no time conversion needed)
+            (
+                self.start_date.format("%Y-%m-%d").to_string(),
+                self.start_time.format("%H:%M").to_string(),
+                self.end_date.format("%Y-%m-%d").to_string(),
+                self.end_time.format("%H:%M").to_string(),
+            )
+        } else {
+            // Convert local date/time to UTC
+            let start_local = Local.from_local_datetime(&self.start_date.and_time(self.start_time)).single();
+            let end_local = Local.from_local_datetime(&self.end_date.and_time(self.end_time)).single();
+            
+            if let (Some(start_dt), Some(end_dt)) = (start_local, end_local) {
+                let start_utc = start_dt.with_timezone(&chrono::Utc);
+                let end_utc = end_dt.with_timezone(&chrono::Utc);
+                (
+                    start_utc.format("%Y-%m-%d").to_string(),
+                    start_utc.format("%H:%M").to_string(),
+                    end_utc.format("%Y-%m-%d").to_string(),
+                    end_utc.format("%H:%M").to_string(),
+                )
+            } else {
+                // Fallback if timezone conversion fails - use local time as-is
+                web_sys::console::warn_1(&"⚠️ Failed to convert local time to UTC, using local time".into());
+                (
+                    self.start_date.format("%Y-%m-%d").to_string(),
+                    self.start_time.format("%H:%M").to_string(),
+                    self.end_date.format("%Y-%m-%d").to_string(),
+                    self.end_time.format("%H:%M").to_string(),
+                )
+            }
+        };
+        
         (
             self.title.clone(),
             self.description.clone(),
-            self.start_date.format("%Y-%m-%d").to_string(),
-            self.start_time.format("%H:%M").to_string(),
-            self.end_date.format("%Y-%m-%d").to_string(),
-            self.end_time.format("%H:%M").to_string(),
+            utc_start_date,
+            utc_start_time,
+            utc_end_date,
+            utc_end_time,
             self.location.clone(),
             self.all_day,
             format!("{:?}", self.status).to_uppercase(),