Add PNG export for charts

- Hover any chart to reveal download button
- Exports with white background and padding
- Works on Dashboard and Comparison pages

src/components/ChartExport.js

@@ -0,0 +1,57 @@
+import React, { useRef } from 'react';
+
+// Wrapper component that adds PNG export to any chart
+export function ExportableChart({ children, filename = 'chart', className = '' }) {
+  const chartRef = useRef(null);
+
+  const exportAsPNG = () => {
+    const chartContainer = chartRef.current;
+    if (!chartContainer) return;
+
+    const canvas = chartContainer.querySelector('canvas');
+    if (!canvas) return;
+
+    // Create a new canvas with white background
+    const exportCanvas = document.createElement('canvas');
+    const ctx = exportCanvas.getContext('2d');
+    
+    // Set dimensions with padding
+    const padding = 20;
+    exportCanvas.width = canvas.width + (padding * 2);
+    exportCanvas.height = canvas.height + (padding * 2);
+    
+    // Fill white background
+    ctx.fillStyle = '#ffffff';
+    ctx.fillRect(0, 0, exportCanvas.width, exportCanvas.height);
+    
+    // Draw the chart
+    ctx.drawImage(canvas, padding, padding);
+    
+    // Export
+    const link = document.createElement('a');
+    link.download = `${filename}-${new Date().toISOString().split('T')[0]}.png`;
+    link.href = exportCanvas.toDataURL('image/png', 1.0);
+    link.click();
+  };
+
+  return (
+    <div className={`exportable-chart ${className}`}>
+      <button 
+        className="chart-export-btn" 
+        onClick={exportAsPNG}
+        title="Download as PNG"
+      >
+        <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"/>
+          <polyline points="7 10 12 15 17 10"/>
+          <line x1="12" y1="15" x2="12" y2="3"/>
+        </svg>
+      </button>
+      <div ref={chartRef} className="chart-canvas-wrapper">
+        {children}
+      </div>
+    </div>
+  );
+}
+
+export default ExportableChart;