feat(analytics): enable Umami's built-in Web Vitals collection

Adds data-performance="true" to the Umami script tag. From v3.1.0+,
this hooks PerformanceObserver and posts LCP, INP, CLS, FCP and TTFB
to the collect endpoint with Google's rating buckets pre-applied.
The Umami dashboard then surfaces them in its built-in Performance
tab with p50/p75/p95 percentiles and per-page / per-device breakdowns
— no custom event taxonomy needed.

Requires the analytics instance to be on Umami ≥ v3.1.0. The
attribute is a no-op on older versions, so no risk to ship before
upgrading the dashboard.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/app/layout.tsx

@@ -78,6 +78,7 @@ export default function RootLayout({
           defer
           src="https://analytics.schoolcompare.co.uk/script.js"
           data-website-id="d7fb0c95-bb6c-4336-8209-bd10077e50dd"
+          data-performance="true"
           strategy="afterInteractive"
         />
       </head>