Move hexgrid heatmap computation to Web Worker

R-tree building, hex grid generation, and percentile sorting now run
off the main thread, eliminating 20s+ UI freezes on large datasets.
The old bundled HexgridHeatmap.js is replaced by a typed worker +
main-thread client with dual R-trees (worker for grid gen, main
thread for synchronous click queries).

frontend/package.json

@@ -41,6 +41,7 @@
     "lucide-react": "^0.515.0",
     "mapbox-gl": "^3.12.0",
     "oidc-client-ts": "^3.2.1",
+    "rbush": "^4.0.1",
     "react": "^19.1.0",
     "react-day-picker": "^9.7.0",
     "react-dom": "^19.1.0",
@@ -60,6 +61,7 @@
     "@types/d3": "^7.4.3",
     "@types/mapbox-gl": "^3.4.1",
     "@types/node": "^24.0.1",
+    "@types/rbush": "^4.0.0",
     "@types/react": "^19.1.2",
     "@types/react-dom": "^19.1.2",
     "@types/turf": "^3.5.32",