feat: implement heat map visualization with Leaflet.heat plugin

- Integrate Leaflet.heat plugin for heat map overlay
- Add heat map toggle control in MapFilters component
- Configure heat map intensity based on report quantity levels (minimal: 0.3, moderate: 0.5, significant: 0.7, severe: 1.0)
- Implement heat map layer management with proper cleanup
- Add custom gradient colors (blue to red) for visual intensity representation
- Configure heat map with radius: 25px, blur: 15, maxZoom: 17
- Add TypeScript type declarations for leaflet.heat
- Heat map respects existing filter state (date range, litter type, quantity)

Implements task 5: Create heat map visualization
Requirements: 3.1, 3.5

src/types/leaflet-heat.d.ts

@@ -0,0 +1,24 @@
+// Type declarations for leaflet.heat plugin
+import * as L from 'leaflet';
+
+declare module 'leaflet' {
+  interface HeatLayerOptions {
+    minOpacity?: number;
+    maxZoom?: number;
+    max?: number;
+    radius?: number;
+    blur?: number;
+    gradient?: { [key: number]: string };
+  }
+
+  interface HeatLayer extends Layer {
+    setLatLngs(latlngs: Array<[number, number, number]>): this;
+    addLatLng(latlng: [number, number, number]): this;
+    setOptions(options: HeatLayerOptions): this;
+  }
+
+  function heatLayer(
+    latlngs: Array<[number, number, number]>,
+    options?: HeatLayerOptions
+  ): HeatLayer;
+}