chore: switch Google Analytics measurement ID to G-9MJF3K866D

Replace GA4 property G-DY6YFW4CTM with G-9MJF3K866D in the root
layout gtag.js snippet (src URL + gtag config).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/layout.tsx

@@ -121,13 +121,13 @@ export default async function RootLayout({
                     }}
                 />
                 <link rel="manifest" href="/manifest.json" />
-                <Script src="https://www.googletagmanager.com/gtag/js?id=G-DY6YFW4CTM" strategy="afterInteractive" />
+                <Script src="https://www.googletagmanager.com/gtag/js?id=G-9MJF3K866D" strategy="afterInteractive" />
                 <Script id="gtag-init" strategy="afterInteractive">
                     {`
                         window.dataLayer = window.dataLayer || [];
                         function gtag(){dataLayer.push(arguments);}
                         gtag('js', new Date());
-                        gtag('config', 'G-DY6YFW4CTM');
+                        gtag('config', 'G-9MJF3K866D');
                     `}
                 </Script>
             </head>