fix: MarkerClusterGroup CJS interop — unwrap .default from import

react-leaflet-cluster uses exports.default instead of module.exports,
so Vite's default import resolves to {default: fn} not fn directly.
Unwrap with ?. default ?? to get the actual component function.

This was never triggered before because the MapContainer only renders
when there are GPS points — previously there were none (GPS bug).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/MapView.jsx

@@ -1,6 +1,7 @@
 import { useEffect, useState, useMemo, Component } from 'react';
 import { MapContainer, TileLayer, Marker, Popup } from 'react-leaflet';
-import MarkerClusterGroup from 'react-leaflet-cluster';
+import _MarkerClusterGroup from 'react-leaflet-cluster';
+const MarkerClusterGroup = _MarkerClusterGroup?.default ?? _MarkerClusterGroup;
 import L from 'leaflet';
 import { fetchMapPhotos } from '../api/photos.js';