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]/SiteLayoutShell.tsx

@@ -0,0 +1,20 @@
+'use client'
+
+import SiteNav from '@/components/dashboard/SiteNav'
+
+export default function SiteLayoutShell({
+  siteId,
+  children,
+}: {
+  siteId: string
+  children: React.ReactNode
+}) {
+  return (
+    <>
+      <div className="w-full max-w-6xl mx-auto px-4 sm:px-6 pt-8">
+        <SiteNav siteId={siteId} />
+      </div>
+      {children}
+    </>
+  )
+}