feat(web): sitewide pre-launch preview banner

Clear notice that the service is not yet open for production use.
Temporary — remove SiteBanner once live.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/components/site-banner.tsx

@@ -0,0 +1,15 @@
+// Temporary pre-launch notice. Remove this component and its import in
+// app/layout.tsx once the service is open for production use.
+export function SiteBanner() {
+  return (
+    <div
+      role="status"
+      className="border-b border-[#4f46e5] bg-[--color-accent] px-4 py-2 text-center text-[12.5px] font-medium leading-snug text-white"
+    >
+      <span className="font-semibold uppercase tracking-wide">Preview</span>
+      <span className="px-1.5 opacity-60">·</span>
+      BuildMyMCPServer is not yet open for production use — sign-ups, server
+      generation and billing are still being finalised, and data may be reset.
+    </div>
+  );
+}