Fix recurring event RRULE INTERVAL and COUNT parameter loss

This commit fixes a critical bug where INTERVAL and COUNT parameters
were being stripped from recurring events during backend processing.

Frontend was correctly generating complete RRULE strings like:
FREQ=WEEKLY;INTERVAL=2;BYDAY=TU,FR;COUNT=6

But backend was ignoring the complete RRULE and rebuilding from scratch,
resulting in simplified RRULEs like:
FREQ=WEEKLY;BYDAY=TU,FR (missing INTERVAL and COUNT)

Changes:
- Modified both events and series handlers to detect complete RRULE strings
- Added logic to use frontend RRULE directly when it starts with "FREQ="
- Maintained backwards compatibility with simple recurrence types
- Added comprehensive debug logging for RRULE generation
- Fixed weekly BYDAY occurrence counting to respect COUNT parameter
- Enhanced frontend RRULE generation with detailed logging

This ensures all RFC 5545 RRULE parameters are preserved from
frontend creation through CalDAV storage and retrieval.

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

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

backend/src/handlers/series.rs

@@ -161,8 +161,16 @@ pub async fn create_event_series(
     // Set priority
     event.priority = request.priority;
 
-    // Generate the RRULE for recurrence
-    let rrule = build_series_rrule_with_freq(&request, recurrence_freq)?;
+    // Check if recurrence is already a full RRULE or just a simple type
+    let rrule = if request.recurrence.starts_with("FREQ=") {
+        // Frontend sent a complete RRULE string, use it directly
+        println!("🎯 SERIES: Using complete RRULE from frontend: {}", request.recurrence);
+        request.recurrence.clone()
+    } else {
+        // Legacy path: Generate the RRULE for recurrence
+        println!("🔄 SERIES: Building RRULE from simple recurrence type: {}", request.recurrence);
+        build_series_rrule_with_freq(&request, recurrence_freq)?
+    };
     event.rrule = Some(rrule);
 
     println!("🔁 Generated RRULE: {:?}", event.rrule);