frontend: Add dropdown for timezone selection

crates/frontend/src/routes/timezones.rs

@@ -0,0 +1,30 @@
+use axum::response::{IntoResponse, Response};
+use headers::{CacheControl, ContentType, HeaderMapExt};
+use http::{HeaderMap, HeaderValue, Method};
+use itertools::Itertools;
+use std::{sync::LazyLock, time::Duration};
+
+static VTIMEZONES_JSON: LazyLock<String> = LazyLock::new(|| {
+    serde_json::to_string(
+        &vtimezones_rs::VTIMEZONES
+            .keys()
+            .sorted()
+            .collect::<Vec<_>>(),
+    )
+    .unwrap()
+});
+
+pub async fn route_timezones(method: Method) -> Response {
+    let mut headers = HeaderMap::new();
+    headers.typed_insert(ContentType::json());
+    headers.insert(
+        "ETag",
+        HeaderValue::from_static(vtimezones_rs::IANA_TZDB_VERSION),
+    );
+    headers.typed_insert(CacheControl::new().with_max_age(Duration::from_hours(2)));
+
+    if method == Method::HEAD {
+        return headers.into_response();
+    }
+    (headers, VTIMEZONES_JSON.as_str()).into_response()
+}