feat: add privacy-friendly Umami page-view analytics (#15)

Cookieless, env-gated page-view tracking via Umami. The <Analytics />
component injects the script only when NEXT_PUBLIC_UMAMI_SRC and
NEXT_PUBLIC_UMAMI_WEBSITE_ID are both set, so local dev and forks send
nothing to our instance. Adds .env.example docs (section 6) and a
homepage footer privacy disclosure. No Cookie consent banner needed.

components/Analytics.tsx

@@ -0,0 +1,18 @@
+import Script from "next/script";
+
+// Privacy-friendly, cookieless page-view analytics (Umami). Both env vars
+// unset → render nothing, so local dev and forks never report to our instance.
+export function Analytics() {
+  const src = process.env.NEXT_PUBLIC_UMAMI_SRC;
+  const websiteId = process.env.NEXT_PUBLIC_UMAMI_WEBSITE_ID;
+  if (!src || !websiteId) return null;
+
+  return (
+    <Script
+      src={src}
+      data-website-id={websiteId}
+      strategy="afterInteractive"
+      defer
+    />
+  );
+}