feat(justine): isolate design system — verbatim CSS + (justine) route group

- Add app/styles/justine/01-homepage.css: rules from 01_homepage.html scoped to [data-justine]
- Replace app/(marketing) with app/(justine): layout wraps data-justine + Plus Jakarta
- JustineHomePage/Nav/Footer: original class names (btn-ink, hero-grid, …) + inline styles from HTML
- Remove app/justine-marketing.css; move /features /pricing /privacy /terms under (justine)

Made-with: Cursor

app/(justine)/page.tsx

@@ -0,0 +1,5 @@
+import { JustineHomePage } from "@/marketing/components/justine/JustineHomePage";
+
+export default function LandingPage() {
+  return <JustineHomePage />;
+}