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)/page.tsx

@@ -1,5 +1,6 @@
 import Link from "next/link";
 import { Button } from "@/components/ui/button";
+import { Navbar } from "./navbar";
 import {
   ChartColumn,
   Zap,
@@ -14,24 +15,7 @@ export default function LandingPage() {
   return (
     <div className="min-h-screen bg-background text-foreground">
       {/* Navbar */}
-      <nav className="border-b border-border bg-card/50 backdrop-blur-sm sticky top-0 z-50">
-        <div className="max-w-6xl mx-auto flex items-center justify-between px-6 py-3">
-          <div className="flex items-center gap-2">
-            <ChartColumn className="w-5 h-5 text-primary" />
-            <span className="font-mono font-bold text-sm tracking-tight">
-              CandleAnnotator
-            </span>
-          </div>
-          <div className="flex items-center gap-3">
-            <Button variant="ghost" size="sm" className="font-mono text-xs" asChild>
-              <Link href="/login">Log in</Link>
-            </Button>
-            <Button size="sm" className="font-mono text-xs" asChild>
-              <Link href="/register">Get Started</Link>
-            </Button>
-          </div>
-        </div>
-      </nav>
+      <Navbar />
 
       {/* Hero Section */}
       <section className="relative overflow-hidden">