Add Google Calendar integration and improve station selection

Implement full OAuth 2.0 flow for Google Calendar, including token
exchange, refresh, and status checks. Add UI for connecting, syncing,
and disconnecting Google accounts in the Calendar panel.

Additionally:
- Make mobile station selection async with error handling and alerts
- Add HAFAS LocMatch method to API client for finding nearest station
- Expose Google OAuth env vars in Next.js config

apps/mobile/src/screens/SettingsScreen.tsx

@@ -92,12 +92,17 @@ export function SettingsScreen({ navigation: _navigation }: ScreenProps) {
     searchTimerRef.current = setTimeout(() => searchStation(text), 400);
   };
 
-  const selectStation = (station: Station) => {
+  const selectStation = async (station: Station) => {
     setOrigin(station);
     setQuery(station.name);
     setResults([]);
-    saveOriginStation(station);
-    rescheduleAllNotifications(); // Recalculate when origin changes
+    try {
+      await saveOriginStation(station);
+      await rescheduleAllNotifications();
+      Alert.alert('Station gespeichert', station.name);
+    } catch {
+      Alert.alert('Fehler', 'Station konnte nicht gespeichert werden.');
+    }
   };
 
   const useCurrentLocation = async () => {
@@ -114,29 +119,40 @@ export function SettingsScreen({ navigation: _navigation }: ScreenProps) {
       const userLat = loc.coords.latitude;
       const userLng = loc.coords.longitude;
 
-      // Find real public-transport stops near the user's GPS coordinates
-      // via the WienerLinien nearby-stops proxy.
-      const stops = await api.findNearbyStops(userLat, userLng, 2000);
-      if (stops.length === 0) {
+      // Try the WienerLinien nearby-stops proxy first
+      let stops: Awaited<ReturnType<typeof api.findNearbyStops>> | null = null;
+      try {
+        stops = await api.findNearbyStops(userLat, userLng, 2000);
+      } catch {
+        // API unavailable, will fall back to HAFAS LocMatch below
+      }
+
+      // If nearby-stops returned results, pick the closest one
+      if (stops && stops.length > 0) {
+        const closest = stops.reduce((best, candidate) => {
+          const bestDist = Math.hypot(best.lat - userLat, best.lng - userLng);
+          const candDist = Math.hypot(candidate.lat - userLat, candidate.lng - userLng);
+          return candDist < bestDist ? candidate : best;
+        }, stops[0]);
+
+        const station: Station = {
+          name: closest.name,
+          extId: closest.id,
+          lat: closest.lat,
+          lng: closest.lng,
+        };
+        await selectStation(station);
+        return;
+      }
+
+      // Fallback: use HAFAS LocMatch directly (same pattern as the web app)
+      const nearestStation = await api.findNearestStationByCoords(userLat, userLng);
+      if (!nearestStation) {
         Alert.alert('Keine Station gefunden', 'Kein ÖPNV-Halt in der Nähe gefunden.');
         return;
       }
 
-      // Pick the closest stop to the user's actual position
-      const closest = stops.reduce((best, candidate) => {
-        const bestDist = Math.hypot(best.lat - userLat, best.lng - userLng);
-        const candDist = Math.hypot(candidate.lat - userLat, candidate.lng - userLng);
-        return candDist < bestDist ? candidate : best;
-      }, stops[0]);
-
-      // Build a Station with the real stop id as extId — HAFAS can look this up.
-      const station: Station = {
-        name: closest.name,
-        extId: closest.id,
-        lat: closest.lat,
-        lng: closest.lng,
-      };
-      selectStation(station);
+      await selectStation(nearestStation);
     } catch (_err) {
       Alert.alert('Fehler', 'Standort konnte nicht ermittelt werden.');
     }