Fix P1/P2 audit issues on LineItem + PackageDetail

- LineItem: add aria-label to info icon for screen readers (P1)
- PackageDetail: stack CTA buttons vertically on narrow screens (P2)
- PackageDetail: section headings use component="h3" for hierarchy (P2)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/molecules/LineItem/LineItem.tsx

@@ -87,6 +87,7 @@ export const LineItem = React.forwardRef<HTMLDivElement, LineItemProps>(
           {info && (
             <Tooltip title={info} arrow placement="top">
               <InfoOutlinedIcon
+                aria-label="More information"
                 sx={{
                   fontSize: 16,
                   color: 'text.secondary',