Clean up unused code and compiler warnings

Removed unused functions and variables identified after RRULE parameter fix:
- Remove unused build_series_rrule function from backend series handler
- Remove unused RecurrenceType::from_rrule and helper functions from frontend
- Prefix unused state variables with underscores to suppress warnings

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

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

backend/src/handlers/series.rs

@@ -460,34 +460,6 @@ pub async fn delete_event_series(
 
 // Helper functions
 
-fn build_series_rrule(request: &CreateEventSeriesRequest) -> Result<String, ApiError> {
-    // Extract frequency from request
-    let recurrence_freq = if request.recurrence.contains("FREQ=") {
-        // Parse RRULE to extract frequency
-        if request.recurrence.contains("FREQ=DAILY") {
-            "daily"
-        } else if request.recurrence.contains("FREQ=WEEKLY") {
-            "weekly"
-        } else if request.recurrence.contains("FREQ=MONTHLY") {
-            "monthly"
-        } else if request.recurrence.contains("FREQ=YEARLY") {
-            "yearly"
-        } else {
-            return Err(ApiError::BadRequest("Invalid RRULE frequency".to_string()));
-        }
-    } else {
-        let lower = request.recurrence.to_lowercase();
-        match lower.as_str() {
-            "daily" => "daily",
-            "weekly" => "weekly",
-            "monthly" => "monthly", 
-            "yearly" => "yearly",
-            _ => return Err(ApiError::BadRequest("Invalid recurrence type".to_string())),
-        }
-    };
-    
-    build_series_rrule_with_freq(request, recurrence_freq)
-}
 
 fn build_series_rrule_with_freq(request: &CreateEventSeriesRequest, freq: &str) -> Result<String, ApiError> {
     let mut rrule_parts = Vec::new();