M2-T09: build data visualization UI (heatmap map as home page)

- self-contained RecordsMap (only module importing leaflet/react-leaflet/
  leaflet.heat/leaflet.markercluster); OSM tiles, swappable behind clean props
- heatmap layers for location + poo (primary); time-range selector fetches
  only the window (locations server-filtered; poo client-filtered)
- toggleable scatter layer with marker clustering; point-select reuses T10's
  edit/delete modals + hooks; query-key prefixes refresh map on mutation
- pure map logic isolated + unit-tested; leaflet mocked in component tests
- responsive layout; typed client only

frontend/src/map/leaflet-heat.d.ts

@@ -0,0 +1,40 @@
+/**
+ * Ambient type declarations for leaflet.heat (no @types package available).
+ *
+ * This file must be a MODULE (has a top-level export) so that `declare module 'leaflet'`
+ * is treated as an AUGMENTATION of the existing leaflet types, not a replacement.
+ * Without the export, the `declare module 'leaflet'` block would shadow all of @types/leaflet.
+ */
+
+// This empty export makes the file a module, enabling proper augmentation semantics.
+export {}
+
+// Augment the 'leaflet' module to add heatLayer and HeatLayer types.
+declare module 'leaflet' {
+  type HeatLatLngTuple = [number, number] | [number, number, number]
+
+  interface HeatLayerOptions {
+    minOpacity?: number
+    maxZoom?: number
+    max?: number
+    radius?: number
+    blur?: number
+    gradient?: Record<number, string>
+  }
+
+  class HeatLayer extends Layer {
+    setLatLngs(latlngs: HeatLatLngTuple[]): this
+    addLatLng(latlng: HeatLatLngTuple): this
+    setOptions(options: HeatLayerOptions): this
+    redraw(): this
+  }
+
+  function heatLayer(latlngs: HeatLatLngTuple[], options?: HeatLayerOptions): HeatLayer
+}
+
+// Declare leaflet.heat as a side-effect-only module.
+declare module 'leaflet.heat' {
+  // Side-effect: augments the Leaflet global with the heatLayer plugin.
+  const _: undefined
+  export default _
+}