UI improvements

frontend/src/hooks/useTravelModes.ts

@@ -0,0 +1,34 @@
+import { useState, useEffect } from 'react';
+import { logNonAbortError } from '../lib/api';
+import type { TransportMode } from './useTravelTime';
+
+interface TravelModeInfo {
+  mode: TransportMode;
+  destinations: number;
+}
+
+/** Fetches which transport modes have precomputed travel time data. */
+export function useTravelModes() {
+  const [availableModes, setAvailableModes] = useState<Set<TransportMode> | null>(null);
+
+  useEffect(() => {
+    const controller = new AbortController();
+
+    fetch('/api/travel-modes', { signal: controller.signal })
+      .then((res) => {
+        if (!res.ok) throw new Error(`HTTP ${res.status}`);
+        return res.json();
+      })
+      .then((data: { modes: TravelModeInfo[] }) => {
+        const modes = new Set<TransportMode>(
+          data.modes.filter((m) => m.destinations > 0).map((m) => m.mode),
+        );
+        setAvailableModes(modes);
+      })
+      .catch((err) => logNonAbortError('travel modes', err));
+
+    return () => controller.abort();
+  }, []);
+
+  return availableModes;
+}