Seitenkarten GL Runde 2: Events, Gassi, Routen + Facade-Erweiterung

- Facade: Polyline (geojson line-source, addTo/setLatLngs/getBounds/remove), clusterGroup,
  marker.getLatLng, map.distance(Haversine), on('click') normalisiert e.latlng aus e.lngLat, _ll objekttauglich
- events: L.markerClusterGroup→UI.map.clusterGroup
- walks: window.L-Guard, L.featureGroup→UI.map.featureGroup, fitBounds ohne .pad
- routes: L.polyline/L.circleMarker→UI.map.*, navMap/Pfeil-Marker→svgMarker, latLngBounds→coords,
  trimMap distance/click, Mini-Vorschauen auf SVG (kein WebGL-Limit, kein OSM-Raster)

backend/static/js/pages/walks.js

@@ -369,7 +369,7 @@ window.Page_walks = (() => {
   }
 
   function _renderMarkers() {
-    if (!_map || !window.L) return;
+    if (!_map) return;
     _markers.forEach(m => m.remove());
     _markers = [];
     _data.forEach(w => {
@@ -386,8 +386,8 @@ window.Page_walks = (() => {
     if (_markers.length === 1) {
       _map.setView(_markers[0].getLatLng(), 13);
     } else if (_markers.length > 1) {
-      const group = L.featureGroup(_markers);
-      _map.fitBounds(group.getBounds().pad(0.2));
+      const group = UI.map.featureGroup(_markers);
+      _map.fitBounds(group, { padding: 50 });
     }
   }