Fix recurring event filtering for month view

Previously filtered events by start date only, which excluded recurring
events that started in previous months/years but have instances in the
current month.

New logic:
- Non-recurring events: filter by exact month match (unchanged)
- Recurring events: include if they could have instances in requested month
- Check event start date is before/during month
- Parse RRULE UNTIL date to exclude expired recurring events
- Let frontend handle proper RRULE expansion

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

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

backend/src/handlers/events.rs

@@ -76,10 +76,54 @@ pub async fn get_calendar_events(
 
     // If year and month are specified, filter events
     if let (Some(year), Some(month)) = (params.year, params.month) {
+        let target_date = chrono::NaiveDate::from_ymd_opt(year, month, 1).unwrap();
+        let month_start = target_date;
+        let month_end = if month == 12 {
+            chrono::NaiveDate::from_ymd_opt(year + 1, 1, 1).unwrap()
+        } else {
+            chrono::NaiveDate::from_ymd_opt(year, month + 1, 1).unwrap()
+        } - chrono::Duration::days(1);
+
         all_events.retain(|event| {
-            let event_year = event.dtstart.year();
-            let event_month = event.dtstart.month();
-            event_year == year && event_month == month
+            let event_date = event.dtstart.date_naive();
+            
+            // For non-recurring events, check if the event date is within the month
+            if event.rrule.is_none() || event.rrule.as_ref().unwrap().is_empty() {
+                let event_year = event.dtstart.year();
+                let event_month = event.dtstart.month();
+                return event_year == year && event_month == month;
+            }
+            
+            // For recurring events, check if they could have instances in this month
+            // Include if:
+            // 1. The event starts before or during the requested month
+            // 2. The event doesn't have an UNTIL date, OR the UNTIL date is after the month start
+            if event_date > month_end {
+                // Event starts after the requested month
+                return false;
+            }
+            
+            // Check UNTIL date in RRULE if present
+            if let Some(ref rrule) = event.rrule {
+                if let Some(until_pos) = rrule.find("UNTIL=") {
+                    let until_part = &rrule[until_pos + 6..];
+                    let until_end = until_part.find(';').unwrap_or(until_part.len());
+                    let until_str = &until_part[..until_end];
+                    
+                    // Try to parse UNTIL date (format: YYYYMMDDTHHMMSSZ or YYYYMMDD)
+                    if until_str.len() >= 8 {
+                        if let Ok(until_date) = chrono::NaiveDate::parse_from_str(&until_str[..8], "%Y%m%d") {
+                            if until_date < month_start {
+                                // Recurring event ended before the requested month
+                                return false;
+                            }
+                        }
+                    }
+                }
+            }
+            
+            // Include the recurring event - the frontend will do proper expansion
+            true
         });
     }