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/pages/MapPage.tsx

@@ -9,6 +9,7 @@ export function MapPage() {
   const [selectedReportId, setSelectedReportId] = useState<string | null>(null);
   const [filters, setFilters] = useState<FilterState>({});
   const [showClusters, setShowClusters] = useState(true);
+  const [showHeatmap, setShowHeatmap] = useState(false);
 
   // Sample data for demonstration (will be replaced with API data in later tasks)
   const sampleReports: Report[] = [
@@ -119,6 +120,10 @@ export function MapPage() {
     setShowClusters(!showClusters);
   };
 
+  const handleToggleHeatmap = () => {
+    setShowHeatmap(!showHeatmap);
+  };
+
   return (
     <div className="h-screen flex flex-col">
       <div className="bg-white border-b border-gray-200 px-4 py-4 sm:px-6 lg:px-8">
@@ -134,6 +139,7 @@ export function MapPage() {
             reports={sampleReports}
             filters={filters}
             showClusters={showClusters}
+            showHeatmap={showHeatmap}
             onMarkerClick={handleMarkerClick}
             onMapClick={handleMapClick}
           />
@@ -143,6 +149,8 @@ export function MapPage() {
             onFiltersChange={setFilters}
             onToggleClusters={handleToggleClusters}
             showClusters={showClusters}
+            onToggleHeatmap={handleToggleHeatmap}
+            showHeatmap={showHeatmap}
           />
         </div>
       </div>