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/App.css

@@ -1774,3 +1774,41 @@ table tbody tr:hover {
     font-size: 1.5rem;
   }
 }
+
+/* Chart Export Button */
+.exportable-chart {
+  position: relative;
+}
+
+.chart-export-btn {
+  position: absolute;
+  top: 8px;
+  right: 8px;
+  z-index: 10;
+  width: 32px;
+  height: 32px;
+  border: none;
+  background: var(--bg-secondary);
+  border-radius: 6px;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--text-secondary);
+  opacity: 0;
+  transition: opacity 0.15s ease, background 0.15s ease;
+}
+
+.exportable-chart:hover .chart-export-btn {
+  opacity: 1;
+}
+
+.chart-export-btn:hover {
+  background: var(--bg-tertiary);
+  color: var(--text-primary);
+}
+
+.chart-canvas-wrapper {
+  width: 100%;
+  height: 100%;
+}