feat: wire frontend state and data flow for multi-chart support

- Add activeChartId/charts state to page.tsx with chart fetching on mount
- ChartSelector integrated in sidebar between header and file upload
- CandleChart and SvgOverlay fetch data scoped by activeChartId
- FileUpload returns chart info, auto-selects new chart after upload
- Annotation create/delete flows include chart_id
- Export scoped by activeChartId
- Toolbox receives activeChartId prop

src/components/FileUpload.tsx

@@ -5,7 +5,7 @@ import { Button } from '@/components/ui/button';
 import { Upload } from 'lucide-react';
 
 interface FileUploadProps {
-  onUploadSuccess: () => void;
+  onUploadSuccess: (chart: { id: number; name: string }) => void;
 }
 
 export default function FileUpload({ onUploadSuccess }: FileUploadProps) {
@@ -36,7 +36,7 @@ export default function FileUpload({ onUploadSuccess }: FileUploadProps) {
           type: 'success',
           text: `Successfully uploaded ${data.count} candle records`,
         });
-        onUploadSuccess();
+        onUploadSuccess(data.chart);
       } else {
         setMessage({
           type: 'error',