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/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 {