feat: responsive system, engineering infra, new components, performance

- Section: responsive prop with media query injection
- visual-weights: fontSize clamp() for responsive sizing
- variables.css: add border-radius/shadow/transition/z-index tokens
- ESLint flat config + Prettier + Changeset init
- New components: Footer, NewsSidebar, BreakingNewsBanner
- Image/Figure: loading=lazy, aspectRatio, sizes props

packages/docs/components/Sidebar.tsx

@@ -1,7 +1,7 @@
 'use client';
 import Link from 'next/link';
 import { usePathname } from 'next/navigation';
-import { cx } from '@newspaperui/utils';
+import { cx } from 'newspaperui-utils';
 
 interface NavItem {
   label: string;