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);

frontend/src/components/sidebar.rs

@@ -2,17 +2,7 @@ use crate::components::CalendarListItem;
 use crate::services::calendar_service::{UserInfo, ExternalCalendar};
 use web_sys::HtmlSelectElement;
 use yew::prelude::*;
-use yew_router::prelude::*;
 
-#[derive(Clone, Routable, PartialEq)]
-pub enum Route {
-    #[at("/")]
-    Home,
-    #[at("/login")]
-    Login,
-    #[at("/calendar")]
-    Calendar,
-}
 
 #[derive(Clone, PartialEq)]
 pub enum ViewMode {

frontend/src/services/calendar_service.rs

@@ -412,7 +412,7 @@ impl CalendarService {
             }
         });
 
-        if let Some(until) = until_date {
+        if let Some(_until) = until_date {
         }
 
         let start_date = base_event.dtstart.date();
@@ -2101,7 +2101,6 @@ impl CalendarService {
         #[derive(Deserialize)]
         struct ExternalCalendarEventsResponse {
             events: Vec<VEvent>,
-            last_fetched: chrono::DateTime<chrono::Utc>,
         }
 
         let response: ExternalCalendarEventsResponse = serde_wasm_bindgen::from_value(json)