Add auth-aware navbar to landing page

- Extract navbar into separate client component (navbar.tsx) with useSession hook
- When authenticated: show "Go to App" button linking to /app
- When unauthenticated: show "Log in" and "Get Started" buttons
- Add SessionProvider to public layout to enable auth hooks
- Create session-provider wrapper component to separate concerns (metadata exports still work)

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

src/app/(public)/layout.tsx

@@ -1,4 +1,5 @@
 import type { Metadata } from "next";
+import { SessionProviderClient } from "./session-provider";
 
 export const metadata: Metadata = {
   title: "Candle Annotator",
@@ -10,5 +11,9 @@ export default function PublicLayout({
 }: Readonly<{
   children: React.ReactNode;
 }>) {
-  return children;
+  return (
+    <SessionProviderClient>
+      {children}
+    </SessionProviderClient>
+  );
 }