feat(web): cluster map markers for large node counts

Wrap MapPage node markers in react-leaflet-cluster (leaflet.markercluster)
with spiderfy-at-max-zoom, zoom-to-bounds on click, clustering disabled at
zoom >= 15, and viewport culling so off-screen markers stay out of the DOM.
Cluster bubbles render a plain themed count badge. Marker label HTML is now
only built when the show-labels toggle is on, halving per-marker DOM weight;
hover-to-reveal no longer applies while labels are toggled off.

package.json

@@ -18,6 +18,7 @@
     "@testing-library/jest-dom": "^7.0.0",
     "@testing-library/react": "^16.3.2",
     "@types/leaflet": "^1.9.17",
+    "@types/leaflet.markercluster": "^1.5.6",
     "@types/node": "^24.0.0",
     "@types/react": "^19",
     "@types/react-dom": "^19",
@@ -37,11 +38,13 @@
     "i18next": "^26.0.0",
     "i18next-browser-languagedetector": "^8",
     "leaflet": "^1.9.4",
+    "leaflet.markercluster": "^1.5.3",
     "react": "^19",
     "react-chartjs-2": "^5",
     "react-dom": "^19",
     "react-i18next": "^17.0.0",
     "react-leaflet": "^5",
+    "react-leaflet-cluster": "^4.1.3",
     "react-markdown": "^10.1.0",
     "react-qr-code": "^2.2.0",
     "react-router": "^8.0.0",