Fix Upcoming card to match grid row height with internal scroll

Replace fixed maxHeight 520px with h-full + overflow-hidden so the
card stretches to match the right column height in the grid row.
The flex chain (Card flex-col → CardContent flex-1 min-h-0 →
ScrollArea flex-1 min-h-0) ensures content scrolls internally
within the row-determined height instead of capping independently.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/dashboard/UpcomingWidget.tsx

@@ -177,7 +177,7 @@ export default function UpcomingWidget({ items }: UpcomingWidgetProps) {
   const dayEntries = Array.from(grouped.entries());
 
   return (
-    <Card className="flex flex-col" style={{ maxHeight: '520px' }}>
+    <Card className="flex flex-col h-full overflow-hidden">
       <CardHeader className="shrink-0">
         <div className="flex items-center justify-between">
           <CardTitle className="flex items-center gap-2">