feat: instant tab navigation by moving SiteNav to shared layout

SiteNav now lives in the [id] layout instead of each page, so it stays
mounted during route transitions. Switching between Dashboard, Uptime,
Funnels, and Settings no longer flashes a full-page skeleton.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/sites/[id]/funnels/new/page.tsx

@@ -91,7 +91,7 @@ export default function CreateFunnelPage() {
   }
 
   return (
-    <div className="w-full max-w-3xl mx-auto px-4 sm:px-6 py-8">
+    <div className="w-full max-w-3xl mx-auto px-4 sm:px-6 pb-8">
       <div className="mb-8">
         <Link 
           href={`/sites/${siteId}/funnels`}