fix(dashboard): grid fills available width instead of fixed 1200px

Wrap GridLayout with WidthProvider (already ships with react-grid-layout, no
new dep) so tiles reflow to actual container width on resize/sidebar-collapse
instead of leaving dead space or overflowing on a hardcoded 1200px canvas.
compactType=vertical already auto-packs the bento-style variable-size tiles.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/Dashboard.jsx

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useMemo, useState } from 'react';
-import GridLayout from 'react-grid-layout';
+import GridLayoutBase, { WidthProvider } from 'react-grid-layout';
+const GridLayout = WidthProvider(GridLayoutBase);
 import 'react-grid-layout/css/styles.css';
 import 'react-resizable/css/styles.css';
 import { AnalyticsApi, LayoutApi, ExportApi, SyncApi } from '../api/client.js';
@@ -137,7 +138,6 @@ export default function Dashboard() {
         layout={visibleLayout}
         cols={12}
         rowHeight={60}
-        width={1200}
         isResizable
         isDraggable
         resizeHandles={['se']}