feat: add Header with Sign In to landing page

- Wrapped LandingPageClient with LandingPageWrapper
- Header now shows Sign In button next to Get Started
- Mobile menu also includes Sign In option
- Fixed Plus Jakarta Sans font import

src/components/landing/LandingPageWrapper.tsx

@@ -321,7 +321,7 @@ export function LandingPageWrapper({ children, className = "" }: WrapperProps) {
     <>
       {/* Google Fonts Import */}
       <style jsx global>{`
-        @import url("https://fonts.googleapis.com/css2?family=Cormorant+Garamond:ital,wght@0,400;0,500;0,600;0,700;1,400&display=swap");
+        @import url("https://fonts.googleapis.com/css2?family=Cormorant+Garamond:ital,wght@0,400;0,500;0,600;0,700;1,400&family=Plus+Jakarta+Sans:wght@400;500;600;700&display=swap");
       `}</style>
 
       <div