Add immediate refresh when new external calendars are created

When users add a new external calendar, events now appear instantly instead
of waiting for the next 5-minute auto-refresh cycle or manual refresh.

Changes:
- Modified ExternalCalendarModal to return newly created calendar ID
- Enhanced on_success callback to immediately fetch events for new calendar
- Added proper visibility checking and error handling
- Removed unused imports to clean up compilation warnings

User experience improvement:
- Before: Add calendar → wait 5 minutes → see events
- After: Add calendar → events appear immediately

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

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

frontend/src/components/external_calendar_modal.rs

@@ -7,7 +7,7 @@ use crate::services::calendar_service::CalendarService;
 pub struct ExternalCalendarModalProps {
     pub is_open: bool,
     pub on_close: Callback<()>,
-    pub on_success: Callback<()>,
+    pub on_success: Callback<i32>, // Pass the newly created calendar ID
 }
 
 #[function_component(ExternalCalendarModal)]
@@ -75,9 +75,9 @@ pub fn external_calendar_modal(props: &ExternalCalendarModalProps) -> Html {
 
             wasm_bindgen_futures::spawn_local(async move {
                 match CalendarService::create_external_calendar(&name, &url, &color).await {
-                    Ok(_) => {
+                    Ok(new_calendar) => {
                         is_loading.set(false);
-                        on_success.emit(());
+                        on_success.emit(new_calendar.id);
                         on_close.emit(());
                     }
                     Err(e) => {

frontend/src/components/sidebar.rs

@@ -1,6 +1,5 @@
 use crate::components::CalendarListItem;
 use crate::services::calendar_service::{UserInfo, ExternalCalendar};
-use chrono::{DateTime, Local, TimeZone, Utc};
 use web_sys::HtmlSelectElement;
 use yew::prelude::*;
 use yew_router::prelude::*;