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/CandleChart.tsx

@@ -42,6 +42,7 @@ interface CandleChartProps {
   selectedColor: string;
   selectedLabelId?: number | null;
   onLabelSelect?: (id: number) => void;
+  activeChartId?: number | null;
 }
 
 export interface CandleChartHandle {
@@ -49,7 +50,7 @@ export interface CandleChartHandle {
 }
 
 const CandleChart = forwardRef<CandleChartHandle, CandleChartProps>(
-  ({ activeTool, onAnnotationChange, selectedColor, selectedLabelId, onLabelSelect }, ref) => {
+  ({ activeTool, onAnnotationChange, selectedColor, selectedLabelId, onLabelSelect, activeChartId }, ref) => {
     const chartContainerRef = useRef<HTMLDivElement>(null);
     const chartRef = useRef<IChartApi | null>(null);
     const seriesRef = useRef<ISeriesApi<'Candlestick'> | null>(null);
@@ -68,7 +69,8 @@ const CandleChart = forwardRef<CandleChartHandle, CandleChartProps>(
     // Fetch candles from API
     const fetchCandles = async () => {
       try {
-        const response = await fetch('/api/candles');
+        const url = activeChartId ? `/api/candles?chartId=${activeChartId}` : '/api/candles';
+        const response = await fetch(url);
         const data = await response.json();
         setCandles(data);
         setIsEmpty(data.length === 0);
@@ -82,7 +84,8 @@ const CandleChart = forwardRef<CandleChartHandle, CandleChartProps>(
     // Fetch annotations from API
     const fetchAnnotations = async () => {
       try {
-        const response = await fetch('/api/annotations');
+        const url = activeChartId ? `/api/annotations?chartId=${activeChartId}` : '/api/annotations';
+        const response = await fetch(url);
         const data = await response.json();
         setAnnotations(data);
         return data;
@@ -308,6 +311,7 @@ const CandleChart = forwardRef<CandleChartHandle, CandleChartProps>(
               body: JSON.stringify({
                 timestamp: nearestCandle.time,
                 label_type: activeTool,
+                chart_id: activeChartId,
               }),
             });
 
@@ -410,6 +414,7 @@ const CandleChart = forwardRef<CandleChartHandle, CandleChartProps>(
           activeTool={activeTool}
           onAnnotationChange={onAnnotationChange}
           selectedColor={selectedColor}
+          activeChartId={activeChartId}
         />
       </div>
     );