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>

frontend/src/components/create_event_modal.rs

@@ -81,17 +81,6 @@ impl Default for RecurrenceType {
     }
 }
 
-impl RecurrenceType {
-    pub fn from_rrule(rrule: Option<&str>) -> Self {
-        match rrule {
-            Some(rule) if rule.contains("FREQ=DAILY") => RecurrenceType::Daily,
-            Some(rule) if rule.contains("FREQ=WEEKLY") => RecurrenceType::Weekly,
-            Some(rule) if rule.contains("FREQ=MONTHLY") => RecurrenceType::Monthly,
-            Some(rule) if rule.contains("FREQ=YEARLY") => RecurrenceType::Yearly,
-            _ => RecurrenceType::None,
-        }
-    }
-}
 
 /// Parse RRULE string into recurrence components
 /// Example RRULE: "FREQ=WEEKLY;INTERVAL=2;BYDAY=MO,WE,FR;UNTIL=20231215T000000Z"
@@ -763,7 +752,7 @@ pub fn create_event_modal(props: &CreateEventModalProps) -> Html {
         })
     };
 
-    let on_priority_input = {
+    let _on_priority_input = {
         let event_data = event_data.clone();
         Callback::from(move |e: InputEvent| {
             if let Some(input) = e.target_dyn_into::<HtmlInputElement>() {