Fix frontend compilation warnings by removing dead code

- Remove unused Route enum and yew_router import from sidebar.rs
- Remove unused last_fetched field from ExternalCalendarEventsResponse
- Prefix unused variables with underscore (_preserve_rrule, _until_date, etc.)

Frontend now compiles cleanly with no warnings.

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

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

frontend/src/app.rs

@@ -914,8 +914,8 @@ pub fn App() -> Html {
                 original_event,
                 new_start,
                 new_end,
-                preserve_rrule,
-                until_date,
+                _preserve_rrule,
+                _until_date,
                 update_scope,
                 occurrence_date,
             ): (
@@ -1474,7 +1474,7 @@ pub fn App() -> Html {
                             let calendar_management_modal_open = calendar_management_modal_open.clone();
 
                             wasm_bindgen_futures::spawn_local(async move {
-                                let calendar_service = CalendarService::new();
+                                let _calendar_service = CalendarService::new();
                                 match CalendarService::get_external_calendars().await {
                                     Ok(calendars) => {
                                         external_calendars.set(calendars);